release-22.1: backupccl: fix off by one index in fileSSTSink file extension

[rhu713]

Backport 1/1 commits from #98041.

/cc @cockroachdb/release

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Currently, the logic that extends the last flushed file fileSSTSink does not trigger if there is only one flushed file. This failure to extend the first flushed file can result in file entries in the backup manifest with duplicate start keys. For example, if the first export response written to the sink contains partial entries of a single key a, then the span of the first file will be a-a, and the span of the subsequent file will always be a-<end_key>. The presence of these duplicate start keys breaks the encoding of the external manifest files list SST as the file path + start key combination in the manifest are assumed to be unique.

Fixes #97953

Release note: None

Release justification: bug fix that may prevent writing of backup manifest files SST

[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