Updates persistence.xmls to 3.0. Re-enables JPA tests. Removes eclips…

[ljnelson]

…elink-maven-plugin in favor of a simpler approach. Corrects documentation and related guides.

Signed-off-by: Laird Nelson [email protected]

[ljnelson]

The bulk of this PR consists of updating all persistence.xml files in the codebase to version 3.0, and, specifically, to use the exact root element as specified in the first <xsd:documentation> element in the Jakarta Persistence 3.0 schema.

Secondarily, this PR removes the eclipselink-maven-plugin and replaces it with an in-VM execution of the Eclipselink-supplied StaticWeave class by way of the exec-maven-plugin.

The eclipselink-maven-plugin actually used the Hibernate static metamodel generator under the covers; this PR now uses it directly during compilation in relevant places. (As a side effect of all this, I confirmed that the Eclipselink static metamodel generator is basically broken with any persistence.xml that is more complicated than a toy example. It should not be—and therefore is not—used.)

To re-enable the apparently previously disabled JPA tests, I had to make sure that H2 is run in legacy mode, which works around eclipse-ee4j/eclipselink#1393, which is still open.

I've updated the relevant examples and documentation to account for these changes.