release-21.2: ui: fix index count on Database Details Page indexes column

[blathers-crl]

Backport 1/1 commits from #73703 on behalf of @lindseyjin.

/cc @cockroachdb/release

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Resolves #72655

Previously, the index count on the Database Details page was misleading
because it referred to the total number of columns that contained an
index rather than the number of indexes. This commit fixes that issue by
counting the unique index names returned in the API response rather than
the total length of the indexes array.

Release note (bug fix): use correct index count on the indexes column on
the Database Details page

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Release justification: Category 2

[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 

[matthewtodd]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @lindseyjin and @maryliag)

[maryliag]

@lindseyjin make sure to add the Release justification and mark the appropriate checkboxes

[lindseyjin]

Done!

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @maryliag)