Allow registering adapters for JsonElement again

[Marcono1234]

Purpose

Fixes #2787

Description

This is done for backward compatibility, however registering the adapter still has no effect because the built-in adapter has higher precedence (like it is the case for multiple years already).

The Javadoc for the GsonBuilder methods has not been adjusted, to discourage users from trying to register adapters for JsonElement.

Registering an adapter for Object is still not allowed, it is less likely there that users have done this and have not noticed that the code has no effect for multiple years since unlike for JsonElement there might not be a canonical implementation for such an adapter, or at least the implementation wouldn't be as simple as for JsonElement.

Checklist

• New code follows the Google Java Style Guide
  This is automatically checked by mvn verify, but can also be checked on its own using mvn spotless:check.
  Style violations can be fixed using mvn spotless:apply; this can be done in a separate commit to verify that it did not cause undesired changes.
• If necessary, new public API validates arguments, for example rejects null
• New public API has Javadoc
  • Javadoc uses @since $next-version$
    ($next-version$ is a special placeholder which is automatically replaced during release)
• If necessary, new unit tests have been added
  • Assertions in unit tests use Truth, see existing tests
  • No JUnit 3 features are used (such as extending class TestCase)
  • If this pull request fixes a bug, a new test was added for a situation which failed previously and is now fixed
• mvn clean verify javadoc:jar passes without errors

[eamonnmcmanus]

Thanks! I'll merge this as soon as the tests pass (assuming they do). Then I think it is about time we had another release.

[Marcono1234]

Should this be backported and released as Gson 2.11.1 without the Java 8 and API changes, to make it easier for users to switch to that version?

CC @jirijanak would migration from Java 7 to Java 8 as minimum version be a problem for you?