Add SpotBugs analysis check

[bact]

Adds a spotbugs job to the build.yml workflow:

• Runs SpotBugs analysis (effort=Max, threshold=High) (SpotBugs is a fork of FindBugs)
• Fails the job if any bug is found

In pom.xml, replaces the unmaintained findbugs-maven-plugin reporting entry with spotbugs-maven-plugin

Choose threshold=High at this pointm as Medium will report too many errors. We can change this later.

This PR will failed until we fix all bugs reported by SpotBugs (currently 4 of them).

To resolve #210

[github-advanced-security]

You are seeing this message because GitHub Code Scanning has recently been set up for this repository, or this pull request contains the workflow file for the Code Scanning tool.

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• The 'Security' tab will display more code scanning analysis results (e.g., for the default branch).
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[bact]

Failed as expected.

Error: SpotBugs found 4 bug(s). See the Checks/Security tab for annotated details.

https://github.com/spdx/Spdx-Java-Library/actions/runs/29547366964/job/87782469020?pr=434#step:7:20