RDF4J 5.1.0 release notes

[aschwarte10]

PR for the release notes of RDF4J 5.1.0

Please feel free to contribute by adding additional commits. Also feel free to adjust / amend / refine the release notes as appropriate, cc @hmottestad , @JervenBolleman

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PR Author Checklist (see the contributor guidelines for more details):

• [ ] my pull request is self-contained
• [ ] I've added tests for the changes I made
• [ ] I've applied code formatting (you can use mvn process-resources to format from the command line)
• [ ] I've squashed my commits where necessary
• [ ] every commit message starts with the issue number (GH-xxxx) followed by a meaningful description of the change