Use symbol for pipeline icon

[janfaracik]

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

Before

After

Testing done

• Pipeline 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
4. Link to relevant issues in GitHub or Jira
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