Resolve "Slack Tag push notifications are not send (only default branch option)"

What does this MR do?

Fixes tag push notifications for ChatNotificationService when 'only default branch' option is enabled

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them

What are the relevant issue numbers?

Closes #45487 (closed)