Use the version from tools/experimental_features.yaml as the current
language version (as specified in kernel).
Use the "enabledIn" from tools/experimental_features.yaml to write out
the major and minor version for enabling features. This replaces the
hard-coding of when nnbd is enabled (in front-end).
Change-Id: I5b5dd7c2bb0c1a412ef1e660dfbe61e227193e1b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/138284
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>