ci: Explicitly build for oldest and newest compliers

[ppannuto]

This splits the "build" CI rule into two: build-latest runs on the newest available OS image with the newest available copmilers, build-legacy runs on the oldest active LTS release and uses the 'default' cross compiler toolchains from that LTS.

There's also two little general CI cleanups tacked on the end that clean up the logs some.

[brghena]

This is fantastic. Definitely good to check both new and old compilers. Thanks!!

[lschuermann]

Any reason not to use the build-matrix feature on GitHub actions to consolidate those two steps? I think testing across different software versions is the intended use case for that.

[ppannuto]

Any reason not to use the build-matrix feature on GitHub actions to consolidate those two steps? I think testing across different software versions is the intended use case for that.

Ignorance of how to better use/configure GitHub actions? :)

It's also different setup/install steps on the different OSes, so it seemed simpler to just have separate jobs, but I have no feelings one way or another on it.

[ppannuto]

Updated to use the build matrix

[lschuermann]

Updated to use the build matrix

Awesome, thanks! Yeah, it's a bit more duplication than I anticipated still (because the setup is pretty different), but it'll be nice that we're less likely to have these two go out of sync.

[lschuermann]

I ported the legacy branch protection rules of this repo into a ruleset, and changed the required status checks, so this should now be able to move into the merge queue. We might need to close and re-open the PR though...

[ppannuto]

I think your third check pointed to the wrong thing (the ci-build-latest job which doesn't exist now, rather than the ci-build (ubuntu-latest) matrix entry). I think I fixed it:

[lschuermann]

@ppannuto Right, missed that! Third time's the charm.