Update Clang in MRBind to 20.

[adalisk-emikhaylov]

Updates the Clang version used by the MRBind Python-bindings generator, bumping it where the platform allows.

Instead of every install/build script reading the flat clang_version.txt (and clang_version_msys2.txt on Windows) directly, version selection is centralized in a new scripts/mrbind/select_clang_version.sh, which returns the Clang version to use for the current platform:

Platform	Clang	Reason
Windows (MSYS2)	20.1.7-1	bumped
Linux (non-Ubuntu)	20	bumped
macOS	18	Clang 20 can't locate the C++ standard library here yet
Ubuntu ≤ 22.04	18	old Boost fails to compile on newer Clang (llvm/llvm-project#59036)
Ubuntu (other, e.g. 24.04)	18	stock spdlog/libfmt hit call to consteval function ... is not a constant expression

All the install scripts (install_deps_*, install_mrbind_*), the Dockerfiles, and the MSYS2 install steps in the Windows and pip-build workflows now call this script instead of reading the version files.

Other changes:

• generate.mk: the flags -Wno-enum-constexpr-conversion and -frelaxed-template-template-args (only needed by older Clang) are moved out of the always-on common_compiler_parser_flags.txt and added conditionally, only for Clang 18 and 19.
• requirements/ubuntu.txt: add lsb-release, needed by the new script to detect the Ubuntu version.
• config.yml: add select_clang_version.sh to the path filter that triggers Linux rebuilds.
• README-generating.md: documentation updated for the new mechanism.

[adalisk-emikhaylov]

This is still useful, please don't delete.