release-23.1.9-rc: backupccl: during restore, do not .Next() any keys in makeSimpleImportSpans

[blathers-crl]

Backport 1/1 commits from #109750 on behalf of @rhu713.

/cc @cockroachdb/release

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Previously, if bulkio.restore.use_simple_import_spans was true during
restore, makeSimpleImportSpans called .Next() on all end keys of its input
file spans in order to handle the fact that these spans are end keys are end
key inclusive. This resulted in some spans having start or end keys that are
not valid for splitting. This patch removes all .Next() calls in
makeSimpleImportSpans, and instead addresses the end key inclusive file spans
by constantly keeping track of all files that do not have their end points
covered as the covering is created, and immediately populating the next cover
entry with these files.

This fixes an issue where a split can be called on an invalid key that's in the
form of someValidKey.Next() during restore. These invalid keys will generally
have a NULL at the end of the key, which will result in an error when calling
EnsureSafeSplits on this split key. Currently errors from EnsureSafeSplits
are ignored, and thus a split will always be attempted on this type of invalid
split key. This split key can land in the middle of a row with column families,
and thus result in failing SQL queries when querying the restored table.

This patch adds some additional testing for backup manifest file entries with
zero sized spans. The previous .Next() called on all file spans meant that
there were no zero sized spans, so backups with these types of files were
under tested.

Informs: #109483

Release note (bug fix): Fixes an issue where a split can be called on an
invalid key that's in the form of someValidKey.Next() during restore
with bulkio.restore.use_simple_import_spans=true. This
split key can land in the middle of a row with column families, and thus result
in failing SQL queries when querying the restored table.

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Release justification: fixes a severe bug in a default-disabled codepath

[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