Allow users to be hard-deleted from the API

What does this MR do?

Adds a ?hard_delete=true parameter to the administrator-only users API, allowing a hard delete to be triggered

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Related to #28694 (closed)

marked the checklist item Changelog entry added, if necessary as completed

marked the checklist item Documentation created/updated as completed

marked the checklist item Documentation created/updated as incomplete

marked the checklist item Documentation created/updated as completed

marked the checklist item Added for this feature/bug as completed

marked the checklist item Conform by the merge request performance guides as completed

marked the checklist item Conform by the style guides as completed

marked the checklist item Branch has no merge conflicts with master (if it does - rebase it please) as completed

changed the description

marked the checklist item Squashed related commits together as completed

mentioned in issue #33215 (closed)

added 1 commit

• a1929af8 - Allow users to be hard-deleted from the API

Compare with previous version

added 130 commits

• a1929af8...1bf76c76 - 129 commits from branch gitlab-org:master
• aa90eea2 - Allow users to be hard-deleted from the API

Compare with previous version