release-24.3: raft: don't panic on defortifying snapshot from new term

[blathers-crl]

Backport 1/1 commits from #133874 on behalf of @nvanbenschoten.

/cc @cockroachdb/release

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Informs #132762.

This commit fixes a bug introduced by 58a9f53. Now that we no longer assume that snapshots are coming from the leader, we were not defortifying the raft leadership when receiving a snapshot. This meant that if a snapshot came from a new term, we would hit an assertion failure, which the new test reproduces.

This commit addresses this bug by distinguishing between messages that are always sent from the leader of the message's term and messages that indicate that there is a new leader of the message's term, even if the message is not from the leader itself.

This is temporary, and can be removed when we address #127349.

Release note: None

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Release justification: needed for leader leases

[blathers-crl]

Thanks for opening a backport.

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[cockroach-teamcity]

This change is