[INFRA] Drop gcc-{7,8}

[marehr]

Description

After release of seqan3 3.1.x, drop the following compiler

• gcc-7
• gcc-8

Add diagnostics to platform.hpp that says:

• if gcc 7 or gcc 8: "seqan3 3.1.x is the last version that supports gcc 7/8, please upgrade your compiler or stay on that version."

Afterthoughts:

• gcc-9: supports template <> concept always_true = true; we can drop SEQAN3_CONCEPT macro
• "support" (don't drop workarounds, don't add new workarounds, because we can't test that) three-months the latest two minor versions of gcc to give the user time to upgrade.
  • and warn if latest version is too old.
• filesystem can be simplified (get rid of std::experimental::file_system and deprecate seqan3/std/filesystem.)
• minor gcc version in other issue: https://github.com/seqan/product_backlog/issues/410

Tasks

• [x] Update CI and drop workarounds and bump version to 3.2 [@eseiler]
• [x] Add diagnostic: if gcc 7 or gcc 8 is used "seqan3 3.1.x is the last version that supports gcc 7/8, please upgrade your compiler or stay on that version."
• [ ] Update documentation that states gcc-7 / 8.
  • [ ] README.md
  • [ ] doc/about/api/index.md
  • [ ] snippets about concept usage
  • [ ] Check if the compiler support is documented somewhere else too (search for gcc7, gcc-7, gcc 7; and gcc8, gcc-8, gcc 8)
• [x] filesystem can be simplified
  • [x] seqan3/std/filsystem should just include <filesystem>
  • [x] deprecate seqan3/std/filesystem (i.e. add deprecation warning)

[MitraDarja]

Workarounds to delete:

• [x] SEQAN3_WORKAROUND_GCC_83328
• [x] SEQAN3_WORKAROUND_GCC_87113
• [x] SEQAN3_WORKAROUND_GCC_90897
• [x] SEQAN3_WORKAROUND_GCC7_AND_8_CONCEPT_ISSUES
• [x] SEQAN3_WORKAROUND_GCC_94967

[eseiler]

Done