release-24.2: storage: set correct key in WriteTooOld error from pebbleMVCCScanner

[blathers-crl]

Backport 1/1 commits from #131220 on behalf of @nvanbenschoten.

/cc @cockroachdb/release

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Noticed while reviewing #131093.

This commit fixes pebbleMVCCScanner to set the correct key for the mostRecentKey field when a locking read encounters multiple keys with more recent timestamps. These keys were added to WriteTooOldError in 52c6b1b, which had previously only been carrying the maximum conflicting timestamp. They are currently only used for debugging.

Before this fix, we would always return the first conflicting key, even though we correctly returned the maximum conflicting timestamp.

I'm planning to backport this to each release branch to avoid any confusing during debugging odysseys, but could be convinced that doing so is not worth the (minimal) risk.

Epic: None
Release note: None

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Release justification: avoids debug confusion.

[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.
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[cockroach-teamcity]

This change is