Clean Up Toolchains and GHA

[Flow86]

To ease up matrix builds, I'll checked all favorited platforms and their compilers / boost versions:

OS	Github runner?	clang/gcc versions	boost version
MacOS	No	clang 14.0.0 (darwin 21.6)	None (*)
macos-12	Yes	clang 14.0.0 (darwin 21.6)	1.81
Debian Bullseye	No	clang 9, 11, 13	1.74
		gcc 9, 10
Ubuntu Focal (20.04)	No	clang 11, 13	1.69, 1.74
		gcc 9, 10
ubuntu-20.04	Yes	clang 10, 11, 12, 16*	1.69, 1.74
		gcc 9, 10
Ubuntu Focal (22.04)	No	clang 11, 12, 13, 14, 15	1.74
		gcc 9, 10, 11, 12
ubuntu-22.04	Yes	clang 12, 13, 14	1.74
		gcc 9, 10, 11, 12
Windows	No	mingw gcc 10	None (*)

*) has to be manually installed/packaged

Clang / G++ Default C++ versions

Compiler	C++ Version
gcc 9	14
gcc 10	14
gcc 11	17
gcc 12	17
clang 9	14
clang 11	14
clang 12	14
clang 13	14
clang 14	14
clang 15	14
clang 16	17

So the resulting runner/GHA matrix is:

Runner / OS	compiler	boost version	c++ version	remarks
macos-12	clang 14	1.81.0	default(14)
ubuntu-20.04	gcc 9 oldest	1.69.0	default(14)	coverage, warnings
	gcc 10	1.69.0	default(14)	also release
	clang 10 oldest	1.69.0	default(14)	also release
	~clang 11~	1.69.0	default(14)	nothing of value
	~clang 12~	1.69.0	default(14)	nothing of value
	clang 16 newest	1.81.0	default(17)
ubuntu-22.04	gcc 11	1.74.0	default(17)
	gcc 12 newest	1.82.0	default(17)	needs boost 1.82.0
	~clang 13~	1.74.0	default(14)	nothing of value
	clang 14	1.74.0	default(14)
jenkins
- osx (x64)	clang 11 (darwin 22.1)	1.81.0	default(14)	boost manually built
- linux (x64)	gcc 10	1.81.0	default(14)	boost from backports
- windows (x86)	mingw gcc 10	1.81.0	default(14)	boost manually built
- windows (x64)	mingw gcc 10	1.81.0	default(14)	boost manually built

Minimum supported versions:

• gcc 9
• clang 10
• boost 1.69
• c++ 14

Default versions:

• gcc 10 (linux/windows)
• clang 11 (macos)
• boost 1.81
• c++ 14