Use full URL of user avatar on webhook push

What does this MR do?

Adds the (only_path: :false) in push_event_payload_service.rb

I didn't add a changelog entry because this MR should be part of !13401 (merged) which already has an entry.

Why was this MR needed?

Pass full URL of user avatar in push webhook payload.

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

What are the relevant issue numbers?

Closes #34339 (closed)