mgr/dashboard: fix CephPGImbalance alert

[aaSharma14]

Multiple CephPGImbalance alerts are being shown, even if the balancer is on and says no optimization is required. This PR intends to fix this issue.

Fixes: https://tracker.ceph.com/issues/55568

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Checklist

• Tracker (select at least one)
  • References tracker ticket
  • Very recent bug; references commit where it was introduced
  • New feature (ticket optional)
  • Doc update (no ticket needed)
  • Code cleanup (no ticket needed)
• Component impact
  • Affects Dashboard, opened tracker ticket
  • Affects Orchestrator, opened tracker ticket
  • No impact that needs to be tracked
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  • Updates relevant documentation
  • No doc update is appropriate
• Tests (select at least one)
  • Includes unit test(s)
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  • No tests

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[aaSharma14]

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