HHH-17612 DefaultRevisionEntity: Illegal argument on static metamodel field injection

[mbellade]

https://hibernate.atlassian.net/browse/HHH-17612
https://hibernate.atlassian.net/browse/HHH-18762

Alternative solution to #9027, instead of creating new mappings for default revision entities we're creating new @MappedSuperclasses types which can be extended by both Envers and user's revision entities. Suggestions on the naming of these types are appreciated.

I also added to the migration guide and cleaned up the Envers user guide chapters about revision entities.

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[sebersole]

I'm not sure DefaultRevisionType is the default of anything. It seems to me the types here ought to be something like:

@MappedSuperclass
public class RevisionMapping {
    ...
}

@Entity
class DefaultRevisionEntity extends RevisionMapping {
    ...
}

[mbellade]

Sure, so I've removed "default" for the two mapped-superclasses that had it in the name, and replaced "type" with "mapping" for all of them. LMK if there's anything else I should change.

[sebersole]

lgtm