Use symbol instead of PNG file

[basil]

Make the user interface more consistent with the rest of Jenkins as in jenkinsci/active-directory-plugin#219.

Testing done

Checked for consistency by viewing the page in the UI.

Submitter checklist

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

[PierreBtz]

lgtm, thanks!

Screenshot for future reference: