[GR-70208] JVMCI: Make ResolvedJavaType return a List<? extends JavaType>

[graalvmbot]

For many implementations of ResolvedJavaType, we want to return a list more specific than List<JavaType>. Thus, the signature should change from:

List<JavaType> getPermittedSubclasses();

to

List<? extends JavaType> getPermittedSubclasses();