switch validate to 22.04 for now (backport #10445)

[mergify]

validate-old-ghcs doesn't work on 24.04, which GHA just updated to; and it uses build artifacts from the earlier steps, so for now the whole thing needs to be downgraded to get thinsg working. We must address this properly later.

Template B: This PR does not modify behaviour or interface

E.g. the PR only touches documentation or tests, does refactorings, 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 #10445 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 85f0f0c has failed:

On branch mergify/bp/3.14/pr-10445
Your branch is up to date with 'origin/3.14'.

You are currently cherry-picking commit 85f0f0c6c.
  (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)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   .github/workflows/validate.yml

no changes added to commit (use "git add" and/or "git commit -a")

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