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

[mergify]

This is an automatic backport of pull request #19226 done by Mergify.
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Changes to be committed:
	modified:   x/staking/keeper/validator_test.go

Unmerged paths:
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	both modified:   CHANGELOG.md
	both modified:   x/staking/keeper/validator.go

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[tac0turtle]

this is state breaking, we would need to revert other versions. I dont think we can backport this

[tac0turtle]

blocking merge to double check assumptions

[alexanderbez]

So the reason I feel safe backporting this is because prior it would error and thus halt a chain. Now, the chain would not halt with such a change.

After our call today, we still need to understand if this truly fixes it. If any missing pieces are discovered, I'll address them in a followup PR to main.

[alexanderbez]

This PR is actually pointless.

1. GetLastValidators isn't even called when returning validator updates
2. The validator update logic is actually correct on 0.50.x and main.