Removes deprecated approvers update endpoint

What does this MR do?

This change removes project's approvers update endpoint. Although this is a change in our public API, this should be acceptable because:

• this endpoint has been deprecated for ~1.5 year
• it's used only by few users - these should be notified about removal

Related to #325609 (closed)

Related to !57344 (merged)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

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