The JDK 21 compiler linter flags potential `this-escape` violations in DGS generated code

When compiling code generated by dgs-codegen using JDK 21 and having all linters enabled using the -Xlint:all compiler argument, then the compiler will raise a warning of a potential this-escape in some generated code.

I've created a minimal reproduction case for a violation in the constructor of a Mutation query, see: https://github.com/Badbond/dgs-codegen-reproduction-case-this-escape. We have also seen the issue being raised for a similar case in a Projection using a static initializer:

public class StateMachineFragmentProjection<PARENT extends BaseSubProjectionNode<?, ?>, ROOT extends BaseSubProjectionNode<?, ?>> extends BaseSubProjectionNode<PARENT, ROOT> {
  {
    getFields().put("__typename", null);
  }

  public StateMachineFragmentProjection(PARENT parent, ROOT root) {
    super(parent, root, java.util.Optional.of("StateMachine"));
  }

See also JDK-8299995.

Environment:

• Maven 3.9.5.
• JDK 21.0.2-tem.
• DGS codegen 6.1.3.

Start with the generated Mutation constructor and Projection initializer, reproducing the warning with JDK 21 and -Xlint:all using the linked minimal case. Trace both outputs back through dgs-codegen; done when the generated code no longer raises the reported this-escape warning in either example.