autovalidate was removed in favor of autovalidateMode in #61648. However, this ended up breaking some things. autovalidate should be brought back and deprecated so as to not break anyone, but still encourage the move to autovalidateMode.

This came out of the discussion in https://github.com/flutter/flutter/pull/61648/files#r490653719.

This issue was created retroactively for PR #66267, which solved the problem, but needs to be cherry-picked into beta.