Simplify GitHub workflow

[df7cb]

Run the regression tests on PG 12 .. 17.

The previous version even compiled PG from scratch for the tests. Use apt.postgresql.org packages instead.

(See https://github.com/df7cb/pg_store_plans/actions/runs/10682916445 for how the result looks like. Will try to send a few more PRs to fix the problems in there.)

[rjuju]

hi Christoph,

I'm trying to resurrect the project, but it's going to take a bit of time. I'm for now trying to make the regression tests pass on pg12 to pg16, but I'm not yet able to do that. I found at least one bug on pg16-, but after that there is still one difference in the regression tests. I also discovered that the CI is running the tests against the project master branch rather than the PR branch. I'm trying to fix all of that but honestly my attempts to do minor changes are mostly a waste of time and I think I will just rewrite the whole workflow from scratch. my still broken attempt is at #41 for more details about the issues I mentioned
I'm planning to cherry pick this commit, or just use it as a base for a whole new workflow. I'm really not familiar with the github workflow so any help is highly appreciated.

[rjuju]

I ended up cherry picking this commit with a few minor changes (notably not testing pg17 as it's not supported yet), along with multiple other fixes to get a green CI at #41.

Let me know if you're ok with the resulting commit and if I can close this PR.

[df7cb]

It's your project so if you think it's good enough, it is. :)