INDATA-193
BACKEND: <https://github.com/supabase/platform/pull/30575>
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<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit
* **Refactor**
* Consolidated read replica limit configuration to provide more
consistent behavior across different compute tiers.
* **Tests**
* Added comprehensive test coverage for read replica eligibility checks
and replica limit calculations based on compute tier.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->