release-23.2.0-rc: Revert "kv: stop encoding or decoding synthetic timestamp bit in/from mvcc keys"

[blathers-crl]

Backport 2/2 commits from #117324 on behalf of @nvanbenschoten.

/cc @cockroachdb/release

---

Fixes #117302.

This commit reverts #105523 for the release-23.2 branch while retaining the two commits on master. In doing so, it resolves the replica inconsistency false-positive described in #117302 between v23.1 and v23.2 binaries.

Master will still need a change like #117304 as part of #101938, but this allows us to avoid spreading the migration to remove synthetic timestamps over two releases (which was not the intention). I'm concerned that there may be other unknown consequences of remaining in this mid-migration state, where we don't encode or decode synthetic timestamps, but where the field is still part of the timestamp proto. Pulling the entire migration forward into v24.1 feels safer.

Release note: None
Release justification: Resolves replica inconsistency false-positive.

[blathers-crl]

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[cockroach-teamcity]

This change is 

[shralex]

Reviewed 6 of 6 files at r1.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @pav-kv, @RaduBerinde, and @sumeerbhola)