Feature: add GCC 11 to settings.yml

[SSE4]

closes: #8885

Changelog: Feature: add GCC 11 to settings.yml.
Docs: omit

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[madebr]

GNU gcc 10.2 and 10.3 also exist
(see https://gcc.gnu.org/)

• shouldn't every major gcc version get its own line?

[memsharded]

Yes, better use the same format, please move each major to its own line.

[memsharded]

Looking good, please check comments.

[memsharded]

Isn't this c++2b at the moment?

[SSE4]

both have the same effect, so doesn't matter

-std=gnu++2b                Conform to the ISO 2023 C++ draft standard with GNU extensions (experimental and incomplete support).  Same as -std=gnu++23.
-std=c++2b                  Conform to the ISO 2023 C++ draft standard (experimental and incomplete support).  Same as -std=c++23.

[memsharded]

Yes, effect is the same, but it is the way to indicate the compiler doesn't have full support for c++23, like the above:

if Version(gcc_version) >= "8":
        v20 = "c++2a"
        vgnu20 = "gnu++2a"

Shouldn't we follow the same approach as above?

[SSE4]

seems like c++17, c++20 and c++23 are still incomplete according to the https://gcc.gnu.org/projects/cxx-status.html

[SSE4]

I've changed c++23 -> c++2b and c++20 -> c++2a as they are incomplete.
not sure if I should touch c++17, it's still incomplete, but we already use that instead of c++1z for GCC 10.

[memsharded]

Yes, better use the same format, please move each major to its own line.