Update DocBook schema to DocBook 5.2.1b1

[Girgias]

This should fix the unintentional BC break from DocBook 5.1 to 5.2, meaning we no longer need to have a custom DocBook schema.

Edit: there is a minor issue in that .rng files no longer have an EOL prior to EOF. Reported upstream.

[jordikroon]

@Girgias 5.2.1 has been released around mid-march. That fixes the EOL. Meaning we could upgrade to that version.
I don't have the expertise to guess what the impact could bring. So I trust you fully on this.

• Fixed the accidental backwards incompatibility with version 5.2 where classsynopsis allowed only a single ooclass, ooexception, or oointerface.
• Fixed the "standard" URIs for the publishers schemas.
• Fixed a typo in the description of the condition attribute.
• Corrected several Schematron rules related to callout cross references.
• Modernized the build, updated the testing framework.