Describe the bug

The CMake build system generated by cmsis-toolbox has inconsistent support for some standards:

• The GNU variants for C standards c17 (gnu17) and c23 (gnu23) are missing. This is unexpected, since the non-GNU variant is supported
• The C++ standards c++20 and gnu++20 are incorrectly mapped to c++17 and gnu++17, leading to confusing behaviour. Either c++20 should be accepted and lead to actual C++20 support being enabled in the compiler, or c++20 should be rejected. Accepting c++20 and silently downgrading to c++17 is confusing.

As an additional enhancement, the following missing language standards should be added:

• c++23
• gnu++23

To Reproduce

Specify language-C: gnu17 or language-C: gnu23 in a .cproject.yml file and attempt to build the project. The build fails with unknown 'language' value 'gnu17' !.

Specify language-CPP: c++20 or language-CPP: gnu++20 in a .cproject.yml file and attempt to build a project containing C++20 features (such as requires clauses). The build fails with C++ syntax errors due to the mode not being C++20.

Expected behavior

Language standard support should be consistent and either correctly passed to the compiler or not implemented.

Environment (please complete the following information):

• cmsis-toolbox 2.6.0
• armclang 6.21.0
• OS: Linux

Additional context

New issue as suggested in #1703.