Resolve "Indicate supported image formats in avatar error"

What does this MR do?

Improves the avatar upload error message by including all image formats that are supported.

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

Why was this MR needed?

As described in #43771 (closed) the current error message ("only images allowed") is not suited when an image file is used that is not supported by GitLab.

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
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #43771 (closed)