Bump the minimum compatibility level to C++20

[tsimonq2]

This pull request is very similar to #83, except this is proactive rather than reactive.

kscreen has bumped its minimum compatibility level to C++20. I am seeing a similar trend across other KDE repositories. You can find the C++20 GCC compatibility table here.

As this is not currently blocking anything, I will start testing rebuilds with this change and wait for a proper +2.

[tsimonq2]

On initial testing, this builds okay.

Some missing symbols for libfm-qt, which seem to be "par for the course":

--- libfm-qt-2.1.0/debian/libfm-qt6-15.symbols	2024-11-05 18:33:08.000000000 +0000
+++ libfm-qt-2.1.0/debian/libfm-qt6-15.symbols	2025-01-03 09:53:09.000000000 +0000
@@ -1,4 +1,4 @@
-# SymbolsHelper-Confirmed: 2.1.0 amd64
+# SymbolsHelper-Confirmed: 2.1.0 amd64 arm64 armhf ppc64el s390x
 libfm-qt6.so.15 libfm-qt6-15 #MINVER#
 * Build-Depends-Package: libfm-qt6-dev
  _ZGVZN9QMetaType21registerConverterImplI5QListI4QUrlE9QIterableI13QMetaSequenceEEEbSt8functionIFbPKvPvEES_S_E10unregister@Base 2.0.2
@@ -1348,28 +1348,9 @@
  _ZNK2Fm9Bookmarks13bookmarksFileEv@Base 0.16.0
  _ZNK2Fm9DeleteJob10metaObjectEv@Base 0.16.0
  _ZNK2Fm9Templates10metaObjectEv@Base 0.16.0
+ (optional=templinst|arch=armhf)_ZNSt10_HashtableINSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEESt4pairIKS5_PKcESaISA_ENSt8__detail10_Select1stESt8equal_toIS5_ESt4hashIS5_ENSC_18_Mod_range_hashingENSC_20_Default_ranged_hashENSC_20_Prime_rehash_policyENSC_17_Hashtable_traitsILb1ELb0ELb1EEEE21_M_insert_unique_nodeEjjPNSC_10_Hash_nodeISA_Lb1EEEj@Base 2.1.0
  (optional=templinst)_ZNSt10_HashtableINSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEESt4pairIKS5_St10shared_ptrIKN2Fm8FileInfoEEESaISD_ENSt8__detail10_Select1stESt8equal_toIS5_ESt4hashIS5_ENSF_18_Mod_range_hashingENSF_20_Default_ranged_hashENSF_20_Prime_rehash_policyENSF_17_Hashtable_traitsILb1ELb0ELb1EEEE4findERS7_@Base 1.1.0
  (optional=templinst)_ZNSt10_HashtableINSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEESt4pairIKS5_St10shared_ptrIKN2Fm8FileInfoEEESaISD_ENSt8__detail10_Select1stESt8equal_toIS5_ESt4hashIS5_ENSF_18_Mod_range_hashingENSF_20_Default_ranged_hashENSF_20_Prime_rehash_policyENSF_17_Hashtable_traitsILb1ELb0ELb1EEEE5clearEv@Base 1.1.0
- (optional=templinst)_ZNSt6vectorI18QRegularExpressionSaIS0_EE17_M_realloc_appendIJS0_EEEvDpOT_@Base 2.0.2
- (optional=templinst)_ZNSt6vectorI6QImageSaIS0_EE17_M_realloc_appendIJS0_EEEvDpOT_@Base 2.0.2
- (optional=templinst)_ZNSt6vectorIN2Fm17BrowseHistoryItemESaIS1_EE17_M_realloc_appendIJS1_EEEvDpOT_@Base 2.0.2
- (optional=templinst)_ZNSt6vectorIN2Fm5MountESaIS1_EE17_M_realloc_appendIJS1_EEEvDpOT_@Base 2.0.2
- (optional=templinst)_ZNSt6vectorIN2Fm6VolumeESaIS1_EE17_M_realloc_appendIJS1_EEEvDpOT_@Base 2.0.2
- (optional=templinst)_ZNSt6vectorIN2Fm8FilePathESaIS1_EE17_M_realloc_appendIJRKS1_EEEvDpOT_@Base 2.0.2
- (optional=templinst)_ZNSt6vectorIN2Fm8FilePathESaIS1_EE17_M_realloc_appendIJRS1_EEEvDpOT_@Base 2.0.2
- (optional=templinst)_ZNSt6vectorIN2Fm8FilePathESaIS1_EE17_M_realloc_appendIJS1_EEEvDpOT_@Base 2.0.2
- _ZNSt6vectorIN2Fm8FilePathESaIS1_EE17_M_realloc_insertIJRKS1_EEEvN9__gnu_cxx17__normal_iteratorIPS1_S3_EEDpOT_@Base 0.16.0
- (optional=templinst|arch=armhf s390x)_ZNSt6vectorINSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEESaIS5_EE17_M_realloc_appendIJRPcEEEvDpOT_@Base 2.0.2
- (optional=templinst)_ZNSt6vectorIPN2Fm12ThumbnailJobESaIS2_EE17_M_realloc_appendIJRKS2_EEEvDpOT_@Base 2.0.2
- (optional=templinst)_ZNSt6vectorIPN2Fm16DirTreeModelItemESaIS2_EE17_M_realloc_appendIJS2_EEEvDpOT_@Base 2.0.2
- (optional=templinst)_ZNSt6vectorIPN2Fm16DirTreeModelItemESaIS2_EE17_M_realloc_insertIJRKS2_EEEvN9__gnu_cxx17__normal_iteratorIPS2_S4_EEDpOT_@Base 2.0.2
- _ZNSt6vectorISt10shared_ptrIKN2Fm12BookmarkItemEESaIS4_EE14_M_insert_rvalEN9__gnu_cxx17__normal_iteratorIPKS4_S6_EEOS4_@Base 0.16.0
- _ZNSt6vectorISt10shared_ptrIKN2Fm12BookmarkItemEESaIS4_EE17_M_realloc_insertIJS4_EEEvN9__gnu_cxx17__normal_iteratorIPS4_S6_EEDpOT_@Base 0.16.0
- (optional=templinst)_ZNSt6vectorISt10shared_ptrIKN2Fm8FileInfoEESaIS4_EE17_M_realloc_appendIJRKS4_EEEvDpOT_@Base 2.0.2
- (optional=templinst)_ZNSt6vectorISt10shared_ptrIKN2Fm8MimeTypeEESaIS4_EE17_M_realloc_appendIJRKS4_EEEvDpOT_@Base 2.0.2
- (optional=templinst)_ZNSt6vectorISt10shared_ptrIN2Fm12TemplateItemEESaIS3_EE17_M_realloc_appendIJS3_EEEvDpOT_@Base 2.0.2
- (arch=amd64 arm64 armhf ppc64el riscv64)_ZNSt6vectorISt10shared_ptrIN2Fm12TemplateItemEESaIS3_EE8_M_eraseEN9__gnu_cxx17__normal_iteratorIPS3_S5_EES9_@Base 1.2.1
- (optional=templinst)_ZNSt6vectorISt4pairISt10shared_ptrIKN2Fm8FileInfoEES5_ESaIS6_EE17_M_realloc_appendIJS6_EEEvDpOT_@Base 2.0.2
  (optional=templinst)_ZNSt8__detail9_Map_baseINSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEESt4pairIKS6_St10shared_ptrIKN2Fm8FileInfoEEESaISE_ENS_10_Select1stESt8equal_toIS6_ESt4hashIS6_ENS_18_Mod_range_hashingENS_20_Default_ranged_hashENS_20_Prime_rehash_policyENS_17_Hashtable_traitsILb1ELb0ELb1EEELb1EEixEOS6_@Base 1.1.0
  _ZNSt8__detail9_Map_baseIjSt4pairIKjSt10shared_ptrIKN2Fm8UserInfoEEESaIS8_ENS_10_Select1stESt8equal_toIjESt4hashIjENS_18_Mod_range_hashingENS_20_Default_ranged_hashENS_20_Prime_rehash_policyENS_17_Hashtable_traitsILb0ELb0ELb1EEELb1EEixERS2_@Base 0.16.0
  _ZNSt8__detail9_Map_baseIjSt4pairIKjSt10shared_ptrIKN2Fm9GroupInfoEEESaIS8_ENS_10_Select1stESt8equal_toIjESt4hashIjENS_18_Mod_range_hashingENS_20_Default_ranged_hashENS_20_Prime_rehash_policyENS_17_Hashtable_traitsILb0ELb0ELb1EEELb1EEixERS2_@Base 0.16.0

So technically, this might just break ABI, but that's probably debatable.

All other LXQt rdeps build without issue