Feat: Update CI

[Totto16]

This fixes two Errors I encountered while working on #145 and #143

• We use the ubuntu-latest github action runners, which used to be ubuntu-24.04 but is going to be switched to ubuntu-24.04 soon (see Ubuntu-latest workflows will use Ubuntu-24.04 image actions/runner-images#10636)
  This produces a ton of warnings:

ubuntu-latest pipelines will use ubuntu-24.04 soon. For more details, see https://github.com/actions/runner-images/issues/10636

in the CI runs, and it's generally a better idea to be explicit about that.

Since we don't use a ton of special features or packages, that may break from that transition, it is a good idea to use the newer one (ubuntu-24.04). If you paid close attention to recent CI runs, some / nearly all already used ubuntu-24.04.

• I saw some docker build warnings while I worked on feat: Add GHC 9.12.1 #145 and maybe they can be reported by hadolint already. So I looked at the used version in the CI and it was outdated, so hopefully some new errors / warnings will be found by the newer action (I doubt it, as my local hadolint installation didn't warn me about these warnings, but lets' see 🤷🏼‍♂️ )

This is actually possible, by making checks a fatal error instead of a warning. See here

I also skipped one check: SecretsUsedInArgOrEnv, since this check detects all GPG KEY ARGdirectives as an error, which they aren't.
I fixed all remaining errors, which only were LegacyKeyValueFormat

[develop7]

Thank you for coming forward, looks good