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

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2. Ensure that the pull request title represents the desired changelog entry
3. Please describe what you did
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5. Link to relevant pull requests, esp. upstream and downstream changes
6. Ensure you have provided tests - that demonstrates feature works or fixes the issue