Quiet pipeline emails

What does this MR do?

In order to receive successful pipeline emails, you can only:

• Switch to custom notification with successful pipelines on

And in order to receive failed pipeline emails, you can:

• Switch to participating or watching
• Switch to custom notification with failed pipelines on

All of the emails will only send to the pipeline's author.

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

Did I miss anything?

Why was this MR needed?

People were annoyed about getting emails for successful pipelines by default.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• 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?

Closes #24845 (closed).