object: align spelling of user admin capabilities with ceph

[pgoron]

Description of your changes:
users & buckets admin capabilities aren't spelled the same
way between rook crd (singular) and ceph (plural). It's a bit
misleading when comparing ceph admin caps and rook users.

See https://github.com/ceph/ceph/blob/main/src/rgw/rgw_common.cc#L2095

Which issue is resolved by this Pull Request:
Resolves #

Checklist:

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• Reviewed the developer guide on Submitting a Pull Request
• Pending release notes updated with breaking and/or notable changes for the next minor release.
• Documentation has been updated, if necessary.
• Unit tests have been added, if necessary.
• Integration tests have been added, if necessary.

[travisn]

It seems odd to have both singular and plural, but better to have the backward compatibility than remove the singular.
@thotz Please also review