Make Jena/RDF4J deps provided, set default Jena to 6

[Ostrzyciel]

• Set the default Jena version to 6 and change it to 5 in CI. This eliminates the constant false alarms about supposed CVEs that we don't have... It also makes it more reasonable to e.g., write benchmarks and tests because by default we benchmark and test latest Jena, not some buggy release from a year ago.
• Make Jena and RDF4J deps "provided" so that users can safely downgrade Jena or RDF4J to some earlier version that they desire. This is what makes it possible to use Jena 5 with Jelly still. It also makes sense because usually Jelly is added as an extra dependency to Java projects, not as the "main thing that pulls in Jena".

[codecov]

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[Ostrzyciel]

I will fix the "required" checks after this PR is approved, that's a GH config issue