Add member: Always return 409 when a member exists

What does this MR do?

It always returns 409 status when a member exists for a source (201 was returned for project).

Why was this MR needed?

It helps consistency in api

Does this MR meet the acceptance criteria?

• Changelog entry added
• 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?

Fixes #20732 (closed)