release-22.1: kvserver: lower score threshold for mvcc GC eligibility

[aliher1911]

Backport 1/1 commits from #92118.

/cc @cockroachdb/release

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Previously GC score which is calculated based on average garbage age and percentage of live data was set to 2 to avoid frequent GC invocations. Value was set high in abundance of caution.
Lowering it to 1 would guarantee that we would always collect something and not cause unnecessary frequent GC runs.
To protect against unexpected situations where GC would be firing too frequently, cooldown period of 2 hours is added. This cooldown will apply from gc score of 1 and is decreased proportionally as score reaches 2 (old threshold).

Release note (performance improvement): GC score triggering mvcc GC run is lowered. GC Score is ratio is computed from mvcc stats and uses ratio of live objects and estimated garbage age to estimate collectability of old data. Reduced score will trigger GC earlier lowering interval between runs 3 times, giving 2 times reduced peak garbage usage at the expense of 30% increase of wasteful data scanning on constantly updated data.

Release justification: Improvement required for existing client running on older version.

Fixes #38251

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is