kvserver: maintain key-span GC thresholds

[ajwerner]

Is your feature request related to a problem? Please describe.

Today each range maintains a GC threshold which covers its entire key span. This can get awkward when we want to merge ranges away after removing tables or indexes which might have had a shorter GC TTL. Today, nothing prevents such merges and such merges can result in unexpected violations of observed TTL behavior. This is also the proximate cause of #31563 and makes #31504 not viable.

Describe the solution you'd like
With the advent of cockroachdb/pebble#1339, we could imagine maintaining multiple GC thresholds for various subspans of a range. Generally these thresholds would converge to a single one, but, upon merge, they wouldn't need to.

Additional context

If we were really crazy, we could eventually leverage this to maintain different GC TTLs for different spans within a range.

Jira issue: CRDB-11766

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