Create release v.0.6.0

Features:

• Make use of std::span instead of gsl::span (this requires at least C++-20).
• Get rid of GSL dependency.
• Removed libClaPP_CXX_FS_LINKER_FLAG.
• Removed compatibility to std::experimental::filesystem.
• Get rid of inline namespaces entirely.
• Bumped clang-format from 11 to 18.
• Bumped clang-tidy support from 11 to 17.
• Reworked the complete GitLab-CI.
• Added cmake-format to GitLab-CI.
• Removed some code-smell detected by clang-tidy (e.g. remove of std::endl, remove of std::is_same<>::value, ...)