Refactor ProjectMembers and GroupMembers controllers

What does this MR do?

1. Refactor ProjectMembers and GroupMembers controllers. Make them look alike.
   • Consistent finder execution
   • Move unrelated logic outside of present_ methods
   • Remove find_members method and move the logic to the index method
   • For consistency with GroupMembers, use params.slice for ProjectMembers too. Extract into filter_params method for both controllers.
2. Move params for MembersFinder under initialize to be consistent with GroupMembersFinder and most of other finders.

Closes #118630 (closed)

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Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content