[release] Change version for 2.8 to 2.8.1
[release] Change version for 2.8 to 2.8.1

2.8 needs another cherry-pick but 2.8.0 has already been released on the
Flutter beta branch. To be on the safe side, bump the version to 2.8.1.

Change-Id: I977113b3272264e903b9150b5185c0eadb769057
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/145581
Reviewed-by: William Hesse <a821cddceae7dc400f272e3cb1a72f400f9fed6d@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk