Raise PHP version to 8.1

[vjik]

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	✔️
Fixed issues	-

[codecov]

Codecov Report

Attention: 8 lines in your changes are missing coverage. Please review.

Comparison is base (5aad36a) 92.09% compared to head (ab16536) 92.17%.

Files	Patch %	Lines
src/Column/CheckboxColumnRenderer.php	70.00%	3 Missing ⚠️
src/Column/RadioColumnRenderer.php	66.66%	3 Missing ⚠️
src/Column/ActionColumnRenderer.php	95.65%	1 Missing ⚠️
src/Column/DataColumnRenderer.php	97.14%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #135      +/-   ##
============================================
+ Coverage     92.09%   92.17%   +0.07%     
+ Complexity      518      457      -61     
============================================
  Files            27       27              
  Lines          1734     1611     -123     
============================================
- Hits           1597     1485     -112     
+ Misses          137      126      -11     

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[what-the-diff]

PR Summary

• PHP Version Updates in Workflow Files
  The versions of PHP used in various workflow files have been updated for compatibility and efficiency purposes. This includes the build, composer-require-checker, rector, and static workflow files.

• Changes in Dependency Versions in composer.json
  The PHP version constraint and the versions of dependencies like rector/rector and vimeo/psalm have been updated in the composer.json file.

• Code Rule Updates in rector.php
  The set of code transformation rules in rector.php was updated to LevelSetList::UP_TO_PHP_81, meaning the code transformations now account for PHP 8.1 specific changes.

• Visibility Changes in Various Files
  The visibility of certain properties and constants in a number of files have been modified to optimize encapsulation, data protection, and overall code robustness.

• Naming Convention Updates in DataColumnRenderer
  Method and property names in the DataColumnRenderer class have switched from camelCase to snake_case to maintain consistency across the codebase.

• Method Parameters and Return Type Updates
  Several methods across multiple classes like DataColumnRenderer, RadioColumnRenderer, and SerialColumnRenderer now include the ColumnInterface type in their parameters for stronger data type assurance. Also, certain methods updated their return type for correctness.

• Addition of Constant Array
  A constant array FILTER_TYPES was added to the DataColumnRenderer class to consolidate the filter types used in the class.

• Reference Updates in DataColumnRenderer Methods
  The references to the filter type in renderFilterInput and renderFilterSelect methods were updated to access the newly introduced FILTER_TYPES array.

• Removal of Redundant Test Class and Method
  The ExceptionTest class and the testContentAttributes method in several test classes were removed due to redundancy or irrelevancy.

• Property Reference Updates in Test Methods
  The references to the $index property were updated in the testContent method in CheckboxColumnTest and RadioColumnTest classes for improved testing reliability.