Modify the boost version to be >= 1.77

[xfangfang]

This commit #929 Add one line: #define BOOST_FILESYSTEM_VERSION 4

According to boost changelog 1.77, version 4 was introduced in version boost 1.77.

Users can select Boost.Filesystem version by defining BOOST_FILESYSTEM_VERSION macro to either 3 or 4 when compiling their code.

But cpr looks for boost >= 1.44

cpr/include/CMakeLists.txt

Lines 55 to 61 in 6b3e353

	# Filesystem
	if(CPR_USE_BOOST_FILESYSTEM)
	find_package(Boost 1.44 REQUIRED COMPONENTS filesystem)
	if(Boost_FOUND)
	target_link_libraries(cpr PUBLIC Boost::filesystem)
	endif()
	endif()

On macOS 14, install the latest boost through homebrew, and with anaconda installed. CPR will incorrectly link boost (1.73) within anaconda during compilation, and report error:

Compiling Filesystem version 3 file with BOOST_FILESYSTEM_VERSION defined != 3

[COM8]

Thanks @xfangfang !