Bump CI compiler versions (and update the docs)

[moxian]

Summary

None

Purpose of change

The intent (at least per the existing docs) is to test the oldest supported compiler versions, and the newest ones. The older compilers are there to ensure that we don't introduce "too new" features into the codebase and break people on the old systems. The newer compilers are there to ensure that the new compilers don't break us (i.e. by introducing a new optional warning that we treat as error due to -Werror).

We fell a bit behind on the newest, so this PR bumps them.

Describe the solution

clang: 12 -> 18
gcc: 11 -> 12*

The gcc would have been bumped to gcc-14, but our ASan tests fail on the latest runner images (ubuntu-24.04) (see comments), so i've bumped to the latest gcc available on ubuntu-22.04 instead.
It would be niceto figure out what's going on, but not today.

Also update comments and add a pretty feature matrix table.

Drive-by change: update macOS job name from "Clang 14, macOS 12" to "macOS 13, Apple Clang 15 " to reflect reality, and put extra emphasis on the fact that this is macOS, and not merely a different linux flavour.
(the ccache key update is not technically neccessary, but it looks prettier when it's synced with the job name)

Describe alternatives you've considered

Testing

CI would self-test. (But also, i tried it in CI in my fork, and it seemed to work at the time).

Additional context

[moxian]

gcc-14+ASan timing out the tests on CI is not a good sign

[moxian]

gcc-14+ASan times out tests consistently after 6 hours. It's not a one-off fluke. As far as I can see this has not been the problem with our current gcc-11 - tests there finish within an hour or so.

Sadly, I don't feel in the mood to debug it right now, so I'll try to go for a middle ground and set it togcc-13 instead, for the short term, and see if that helps.

[moxian]

gcc-13 doesn't work (same symptoms)
nor does gcc-12

Is it not a question of gcc version but rather that of OS ? 🤔

I also haven't noticed until just now that we have a special handling of clang-12 which can now be removed? question mark?
Further research required...

[moxian]

It's the OS.
For gcc-12:
22.04 works - https://github.com/moxian/Cataclysm-DDA/actions/runs/13176636773/job/36819050904
24.04 times out - https://github.com/moxian/Cataclysm-DDA/actions/runs/13176636773/job/36819051070
(ignore the job names - I messed up there. See the runner image in the Set up job step and the compiler name in the build)