release-22.1: backupccl: Backup LATEST file is no longer overwritten

[blathers-crl]

Backport 1/1 commits from #77284 on behalf of @DarrylWong.

/cc @cockroachdb/release

---

This change teaches backup to no longer overwrite or delete the LATEST file,
and instead write a new version side by side with the old ones. Other functions are
taught to look for these new LATEST files. To make sure that older backup chains
are not broken by this change,we still maintain an overwritten,
non versioned LATEST file in the base directory if it is a mixed cluster. This
logic can be removed in a post 22.1 release when we no longer write to the base directory.

Release note (enterprise): LATEST files are no longer overwritten and now versioned
and written in the /metadata/latest directory for non mixed clusters.

Release justification: Low risk change to where files are written for a highly requested feature.

[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