UUID-based identities search is very slow

[emyller]

Warning

This issue aims to resolve a hindering performance issue.

A GIN index for identities exists in production, and it causes severe database degradation under certain circumstances observed in production, e.g. searching for an UUID identifier.

Timeline for context:

1. A GIN index identity_identifier_idx was added to environments_identity.identifier.
   feat: Add GIN index on Identity.identifier #5369
2. The migration was reverted before it reached production.
   fix: Revert "feat: Add GIN index on Identity.identifier" #5481
3. Regardless, the index exists. It might have been added manually.

Acceptance criteria

• The GIN index is deleted so it doesn't derail a query plan.
• Rate limiting is added to the identity search endpoint

[matthewelwell]

Slack thread with additional context: https://flagsmith.slack.com/archives/C02HPR34L65/p1789398032582419