[NOSQUASH] Use cpp17, and some cleanups

[Desour]

Descriptions for each commit:

• Bumps the mimimum gcc and clang versions to the minimum that ubuntu 20.04 supports (gcc 7 and clang 6). And re-enables the CI builds.
• Bumps the C++ std to 17. (Mostly supported by gcc 7 and clang 6, see https://en.cppreference.com/w/cpp/compiler_support/17.)
• Fixes a bug where try_lock is called without using the return value. (Found because of [[nodiscard]].)
  Note: The Thread logic still looks faulty to me, i.e. I'm not sure if there's a race condition if the try_lock fails, or if we sometimes call try_lock on a mutex locked by the current thread (which is UB). Someone should fix or explain this at some point.
• Our Optional type is replaced with std::optional.
• Forward declarations of the form namespace foo { namespace bar { ... }} are replaced by the new syntax namespace foo::bar { ... }.
• Our STATIC_ASSERT macro is replaced by static_assert. (This is not C++17 specific, and could've been done much earlier.)
• The -Wimplicit-fallthrough warnings are no longer disabled. The [[fallthrough]] attribute is now guaranteed to exist. (Btw. we can easily use attributes of newer C++ standards, see also feature testing.)
  (Note: The same could be done for the unused-parameter warnings, with [[maybe_unused]]. But there seem to be many irrlicht functions triggering the warning, and it doesn't seem worth it.)
• [[noreturn]] is used, as opposed to the compiler-specific syntax. (Also not new in C++17.)

(Related: #13583)

Related (no dependency, but you'll get deprecation warnings without): minetest/irrlicht#202

To do

This PR is a Ready for Review.

How to test

Build.

[lhofhansl]

Nice. Looks good. The one extra break looks weird, unless that was a (glaring) bug before.

[sfan5]

LGTM otherwise

[numberZero]

LGTM