release-24.1: backupccl: don't attempt to set GC TTL on NonPhysical table

[blathers-crl]

Backport 1/1 commits from #123202 on behalf of @stevendanna.

/cc @cockroachdb/release

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In #88342, we introduced code to reduce the GC TTL of tables to facilitate
faster cleanup in the case of a failed restore. This introduced a bug
in which the revert could fail because we are not able to set the
GC.TTL on non-physical tables.

Here, we exclude non-physical tables when lowering gc.ttlseconds and
also make this a warning instead of an error.

For releases where we've both moved to only inspecting the namespace
table for RESTORE collisions and where the default TTL is low, we
should consider removing this code completely.

Epic: none
Fixes #122168

Release note (bug fix): Fix a bug where a failed RESTORE could leave
the system in a state where re-attempting the restore was not possible
without manual intervention.

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Release justification: Bug fix for a bug that causes descriptors to be permanently offline without manual intervention.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
• Backports should not break backwards-compatibility.
• Backports should change as little code as possible.
• Backports should not change on-disk formats or node communication protocols.
• Backports should not add new functionality (except as defined
  here).
• Backports must not add, edit, or otherwise modify cluster versions; or add version gates.
• All backports must be reviewed by the owning areas TL and one additional
  TL. For more information as to how that review should be conducted, please consult the backport
  policy.

If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• 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. State changes must be further protected such that nodes running old binaries will not be negatively impacted by the new state (with a mixed version test added).
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.
• Your backport must be accompanied by a post to the appropriate Slack
  channel (#db-backports-point-releases or #db-backports-XX-X-release) for awareness and discussion.

Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is