Dismiss unsafe class alerts about org.apache.commons.digester3.Digester

[mig42]

This PR dismisses some security alerts about using potentially unsafe classes:

• https://github.com/jenkinsci/plasticscm-plugin/security/code-scanning/9
• https://github.com/jenkinsci/plasticscm-plugin/security/code-scanning/10
• https://github.com/jenkinsci/plasticscm-plugin/security/code-scanning/11
• https://github.com/jenkinsci/plasticscm-plugin/security/code-scanning/12

They're warning about the usage of org.apache.commons.digester3.Digester, which has uses org.xml.sax.XMLReader underneath and it's therefore vulnerable to XML eXternal Entity injection (XXE).

The plugin builds the Digester via DigesterUtils, which already applies the protections in https://cheatsheetseries.owasp.org/cheatsheets/XML_External_Entity_Prevention_Cheat_Sheet.html#xmlreader .

We can safely dismiss this alerts, as we're already protecting our XML readers.

Testing done

Not needed. This PR only adds @SuppressWarnings annotations.

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