release-24.1: raft: re-enable config change safety

[blathers-crl]

Backport 2/2 commits from #124804 on behalf of @pav-kv.

/cc @cockroachdb/release

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Config changes in this raft implementation require a safety constraint: the leader must not append a config change if it hasn't applied all config changes in its log.

The DisableConfChangeValidation flag disables this check under the assumption that the state machine layer provides the equivalent guarantee. However, it is hard to argue that this is true in split leaseholder/leader scenarios.

This commit re-enables this check, to bring the safety back. The other two state-machine-level checks concerned with entering and leaving joint configs can still be disabled.

Related to #105797, etcd-io/raft#81, #106515 (#106515 (comment))

Epic: none
Release note: none

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Release justification: safety bug fix

[cockroach-teamcity]

This change is 

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

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  issues or test-only changes.
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Also, please add a brief release justification to the body of your PR to justify this
backport.

[nvanbenschoten]

Do we need to get this back to v23.2 in one form or another as well?

[pav-kv]

I think so, this behaviour was changed last summer.