Raise the minimal XWiki version to 16.10

The PlantUML library used by the macro (1.2025.2, since PlantUML Macro 2.4) depends on the org.eclipse.elk:* artifacts (used for the ELK layout), whose POMs declare an eclipse-plugin packaging. The XWiki Extension Manager only supports this packaging since XWiki 16.4.0 (XCOMMONS-3003: "Failed to validate extension that depends on com.atlassian.sal:sal-api"; its backport to 15.10.10+ doesn't include the eclipse-plugin artifact handler). On older XWiki versions, installing the macro from a Maven repository fails with:

ComponentLookupException: Can't find descriptor for the component with type [interface org.xwiki.extension.handler.ExtensionHandler] and hint [eclipse-plugin]

This was verified with the new functional tests, on XWiki 14.10 and 15.10.15 (fails) and 16.10.0 (works). XWiki 16.10 is the first LTS supporting it, hence the new minimal version. It also covers the Java 11 requirement of PlantUML since 1.2026.2 (XWiki 16.10 requires Java 17).

• Raise the parent POM to the XWiki 16.10 contrib parent
• Use the generic resource reference serializer to compute the URL of the generated images (the standard/tmp serializer isn't available anymore)
• Replace the removed setDefaultCategory() API