fix(x/staking): Refactor GetLastValidators (backport #19226)

[mergify]

This is an automatic backport of pull request #19226 done by Mergify.
Cherry-pick of a69836b has failed:

On branch mergify/bp/release/v0.47.x/pr-19226
Your branch is up to date with 'origin/release/v0.47.x'.

You are currently cherry-picking commit a69836b3b.
  (fix conflicts and run "git cherry-pick --continue")
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Changes to be committed:
	modified:   x/staking/keeper/validator_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   CHANGELOG.md
	both modified:   x/staking/keeper/validator.go

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

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[tac0turtle]

this is consensus breaking, it could leave the network in a halt due to apphashes differing. we should do a small informational advisory

[tac0turtle]

blocking merge to double check assumptions

[alexanderbez]

blocking merge to double check assumptions

#19230 (comment)

[tac0turtle]

should we close this pr?

[alexanderbez]

we can close