release-22.2: ui/cluster-ui: fix no most recent stmt for active txns

[blathers-crl]

Backport 1/1 commits from #88179 on behalf of @xinhaoz.

/cc @cockroachdb/release

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Fixes #87738

Previously, active txns could have an empty 'Most Recent Statement' column, even if their executed statement count was non-zero. This was due to the most recent query text being populated by the active stmt, which could be empty at the time of querying. This commit populates the last statement text for a txn even when it is not currently executing a query.

This commit also removes the isFullScan field from active txn pages, as we cannot fill this field out without all stmts in the txn.

Release note (ui change): Full scan field is removed from active txn details page.

Release note (bug fix): active txns with non-zero
executed statement count now always have populated stmt text, even when no stmt is being executed.

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Release justification: Bug fixes and low-risk updates to new functionality

[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 

[maryliag]

Reviewed 3 of 5 files at r1.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @xinhaoz)