improvement: replace (un)likely macro with C++20 attribute

[SamantaTarun]

resolves #1514

[heinezen]

Looks good so far :)

Now you only need to pass the sanity checks.

• Update the copyright years. You can run python3 -m buildsystem.codecompliance --all --fix in the repo to let the buildsystem do this for you.
• Add you name/username/commit email to copying.md
• You can check if everything's correct by running make checkall before pushing

[heinezen]

Also, you can remove the macros in line 52 and 55 in the file below since they're not needed anymore:

openage/libopenage/util/compiler.h

Lines 52 to 55 in c3d7394

	#define unlikely(x) __builtin_expect(!!(x), 0)
	#else
	#define likely(x) (x)
	#define unlikely(x) (x)

[heinezen]

Great!