refactor(x/staking): Migrate Validators to use Collections

[likhita-809]

Description

Closes: #16377

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Author Checklist

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• confirmed the correct type prefix in the PR title
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• reviewed state machine logic
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• manually tested (if applicable)

[tac0turtle]

left two questions otherwise looks good

[julienrbrt]

lgtm! one doubt.

[atheeshp]

mostly lgtm