Promote mz_cluster_replica_sizes from mz_internal to mz_catalog

[chaas]

See https://github.com/MaterializeInc/materialize/issues/26026. We will need to coordinate this rollout with:

• Docs changes
• Console
• Cloud prometheus exporter
• mzadmin
• Demos

(see full list on tracking ticket). Also needs a release note, since this may break users' queries (permitted because this is a catalog table in mz_internal).

Motivation

• This PR adds a known-desirable feature. https://github.com/MaterializeInc/materialize/issues/26026

Tips for reviewer

• Do we need a new/different OID for the table and index, or can we reuse the existing one?
• Do we need any sort of migration for this?

Checklist

• [x] This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• [ ] This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• [ ] If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• [ ] If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• [x] This PR includes the following user-facing behavior changes:
  • Move the mz_cluster_replica_sizes system catalog table from the mz_internal schema to the mz_catalog schema, making the table definition stable. Any queries referencing the mz_internal.mz_cluster_replica_sizes catalog table will now need to use mz_catalog.mz_cluster_replica_sizes.