refactor(x/staking): migrate ValidatorUpdates to collections

[likhita-809]

Description

Closes: #16373

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Author Checklist

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• included the correct type prefix in the PR title
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• followed the guidelines for building modules
• included the necessary unit and integration tests
• added a changelog entry to CHANGELOG.md
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• confirmed the correct type prefix in the PR title
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• reviewed state machine logic
• reviewed API design and naming
• reviewed documentation is accurate
• reviewed tests and test coverage
• manually tested (if applicable)

[julienrbrt]

lgtm! However changelog is missing.

[atheeshp]

there is a failing test.