Remove support for Stagehand templates #4417

Many CI tests are failing because we have tests for Stagehand but it no longer works on the latest dev SDKs (because it doesn't support null safety).

The package has been discontinued for a long time in favour of "dart create" so we should remove support here (and associated tests) and just provide a message if someone invokes Dart: New Project on a pre-Dart 2.10 SDK.