release-20.2: security: allow colons in cert-principal-map principals

[blathers-crl]

Backport 1/1 commits from #67703 on behalf of @bobvawter.

/cc @cockroachdb/release

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Customers whose internal certificate authority issues certificates with colons
in the principal name were unable to use the cert-principal-map flag.

This change makes the flag-parsing logic split each mapping on the last colon
in the mapping. This does not change the interpretation of existing uses of the
flag, but allows principal names with colons to be used without any need for
escape sequences.

Fixes #66435

Release note (cli change): The cert-principal-map flag now allows the
certificate principal name to contain colons.

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Release justification: low risk, high benefit changes to existing functionality

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues.
• 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.

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