Display project icon from the default branch instead of master

What does this MR do?

This MR makes sure that the project icon is being read from the currently selected default branch instead of master.

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

I'm currently always invalidating the avatar cache when someone changes the default branch.

Why was this MR needed?

There are cases where no master branch exists.

What are the relevant issue numbers?

closes #20774 (closed)

Screenshots (if relevant)

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 style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together