Default ray_enabled to False #4315

This ends the compatibility window for undeclared servers, step 6 of #3669.

• ray_enabled defaults to False, and only servers whose class literally declares True make gym start Ray.
• Servers that don't declare Ray launch with RAY_ENABLE_AUTO_CONNECT=0. An undeclared server that calls Ray then fails with "Ray has not been started yet", instead of starting a private cluster in each process.
• This breaks external servers that use Ray without declaring it, so it needs a release note.

PR: #4307, open and stacked on #4301.