Rename projects to have .project extension (backport #9958)

[mergify]

Fixes #9925, renaming project files to have a .project extension.

Template Β: This PR does not modify cabal behaviour (documentation, tests, refactoring, etc.)

Include the following checklist in your PR:

• Patches conform to the coding conventions.
• Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions).

---

This is an automatic backport of pull request #9958 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of d8147f6 has failed:

On branch mergify/bp/3.12/pr-9958
Your branch is up to date with 'origin/3.12'.

You are currently cherry-picking commit d8147f62c.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   .github/workflows/validate.yml
	modified:   .gitlab/ci.sh
	modified:   CONTRIBUTING.md
	modified:   README.md
	modified:   bootstrap/generate_bootstrap_plans
	modified:   cabal-testsuite/README.md
	renamed:    cabal.project.meta -> cabal.meta.project
	renamed:    cabal.project.release -> cabal.release.project
	renamed:    cabal.project.validate.libonly -> cabal.validate-libonly.project
	renamed:    cabal.project.validate -> cabal.validate.project
	modified:   project-cabal/README.md
	modified:   project-cabal/ghc-latest.config
	modified:   validate.sh

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   .github/workflows/quick-jobs.yml
	both modified:   Makefile
	deleted by us:   cabal.bootstrap.project

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[Mikolaj]

Thank you.

[ulysses4ever]

maybe we should first backport #9845
at least, this is the order I'm seeing when looking at the history of quick-jobs.yml: https://github.com/haskell/cabal/commits/master/.github/workflows/quick-jobs.yml

created that backport: #10041 but it fails due to the update of the runners to ghc 9.10. So, we'd need to first land another #10027 (backport of #10026) but it's quite far away from what's currently on 3.12, so the best course would be if someone could manually reapply the fix from #10026 to 3.12 and then.... oh, my.