Deprecate `DefaultEntityResolver.resolveEntity(String)` for removal

[carlosame]

Currently, the little-used DefaultEntityResolver.resolveEntity(String) method uses the DocumentTypeDeclaration class, which carries an external dependency to jclf-text.

Removing that method would make DefaultEntityResolver a bit lighter, and free from external dependencies. Somehow, the method already had a comment that it was "deprecated" but did not have any @Deprecated annotation.

The plan is:

1. Deprecate for removal in 3.5.2.
2. Remove in 4.0.0.

If you could be impacted by the removal of the method, please post a comment here.