Use proper STI names in code for various namespace types

What does this MR do and why?

Convert usage of hardcoded namespace types such as Group and User into their STI equivalent names. For example, type: 'Group' would become type: Group.sti_name

This is because we are now overriding the STI name for namespaces (for example https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/models/namespaces/user_namespace.rb). So we should really use those values rather than hardcoding them.

MR acceptance checklist

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Related to #341063 (closed)