release-21.2: rpc: restore 20s connect timeout for grpc dialer

[blathers-crl]

Backport 1/1 commits from #71417 on behalf of @aliher1911.

Release justification: Without this fix, client on slow network can not upgrade past v20.1

/cc @cockroachdb/release

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Changes in #49114 lead to default connect timeout value
dropping to 0 instead of default 20s.
This is not always enough for multiregion clients with hub spoke
network topologies as connections don't have enough time to initially
go through.
This change restores initial value that matches grpc defaults.

Release note (bug fix): Connect timeout for grpc connections is
set to 20s to match pre 20.2 default value.

Fixes #71411

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Release justification:

[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.
• 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