Use symbol for multi branch pipeline icon

[janfaracik]

Follow up to @mawinter69 changes in core (jenkinsci/jenkins#9127). This PR uses the branch symbol now that the 'New item' page supports displaying them.

Before

After

Testing done

• Branch symbol displays as expected in the 'New item' page

### Submitter checklist
- [x] Make sure you are opening from a **topic/feature/bugfix branch** (right side) and not your main branch!
- [x] Ensure that the pull request title represents the desired changelog entry
- [x] Please describe what you did
- [x] Link to relevant issues in GitHub or Jira
- [x] Link to relevant pull requests, esp. upstream and downstream changes
- [x] Ensure you have provided tests - that demonstrates feature works or fixes the issue