release-21.2: kvserver: don't reject raft snapshots on draining nodes

[aayushshah15]

Backport 1/1 commits from #77246.

/cc @cockroachdb/release

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Previously, draining nodes were incorrectly rejecting all snapshots --
including Raft snapshots. This meant that the replicas on those draining nodes
that needed Raft snapshots to catch up would never be able to do so. This
could've lead to tacit unavailability where, even in cases where all the
replicas are live, if a majority is on draining nodes, the range would be
stalled.

Discovered in cockroachlabs/support#1459

Release justification: bug fix

Release note (bug fix): Previously, draining nodes in a cluster without
shutting them down could stall foreground traffic in the cluster. This patch
fixes this bug.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is