This is the cheap version of #70427.

We can keep the current schema GC system around, but instead of dropping a ClearRange we drop a DeleteRange with an MVCC range tombstone (requires #70415 first). It will then get cleaned up by MVCC GC at some point. This should only be done when a TBD experimental cluster setting and version gate is enabled, which it won't be by default. This will at least preserve MVCC guarantees, enabling incremental tenant backups and streaming cluster replication.

This will be ineffectual until we implement MVCC range tombstone GC in #70414, but we can get the plumbing in place at least. The cluster setting will be marked as experimental anyway, at least to begin with.