Update ontology metadata

[RinkeHoekstra]

Curent PAV metadata refers to URLs (mindinformatics) that no longer exist.

These should be updated as part of a patch, or new release.

[RinkeHoekstra]

The following:

        <owl:versionIRI rdf:resource="&pav;2.3"/>
        <owl:priorVersion rdf:resource="&pav;2.2"/>
        <owl:backwardCompatibleWith rdf:resource="&pav;2.2"/>
        <owl:backwardCompatibleWith rdf:resource="&pav;2.1"/>
        <owl:backwardCompatibleWith rdf:resource="&pav;2.0/"/>
        <owl:backwardCompatibleWith rdf:resource="&pav;authoring/2.0/"/>
        <owl:backwardCompatibleWith rdf:resource="&pav;provenance/2.0/"/>
        <owl:backwardCompatibleWith rdf:resource="&pav;versioning/2.0/"/>

Dereference to RawGit URLs which has reached EOL on October 2019. The service may be discontinued at any moment. See https://rawgit.com/.

These should be corrected to point at deeplinks into the GitHub releases.