storage: set correct key in WriteTooOld error from pebbleMVCCScanner

[nvanbenschoten]

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

[cockroach-teamcity]

This change is 

[tbg]

Nice find.

[nvanbenschoten]

TFTR!

bors r+

[craig]

Build succeeded:

• docker_image_amd64
• unit_tests
• macos_amd64_build
• acceptance
• linux_amd64_build
• local_roachtest_fips
• linux_arm64_build
• lint
• local_roachtest
• macos_arm64_build
• check_generated_code
• linux_amd64_fips_build
• windows_build
• examples_orms

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
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You might need to create your backport manually using the backport tool.

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error creating merge commit from 6a935de to blathers/backport-release-23.1-131220: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 23.1.x failed. See errors above.

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error creating merge commit from 6a935de to blathers/backport-release-23.2-131220: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 23.2.x failed. See errors above.

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