release-21.2: server: CancelSession propagates gateway metadata

[matthewtodd]

Backport 1/1 commits from #75814.

/cc @cockroachdb/release

---

Fixes #75758.

Previously, HTTP session cancelation requests from the DB Console were
routed to the proper node without passing along the gateway metadata
necessary for authentication. This enabled users without the
CANCELQUERY option to cancel the sessions of other users, which they
should not have had permission to do.

This commit addresses that issue by properly propagating that gateway
metadata in the CancelSession endpoint.

Release note (bug fix): The CancelSession endpoint now correctly
propagates gateway metadata when forwarding requests.

---

Release justification: Category 3: Fixes for high-priority or high-severity bugs in existing functionality

[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 

[maryliag]

Reviewable status: complete! 1 of 0 LGTMs obtained