Disable Dart 3 CI run for now

Author: greglittlefield-wf

.github/workflows/dart_ci.yml

@@ -15,8 +15,8 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        # Can't run on `beta` (Dart 3) until we're fully null-safe.
-        sdk: [2.18.7, stable]
+        # Can't run on `stable` (Dart 3) until we're fully null-safe.
+        sdk: [2.18.7]
     steps:
       - uses: actions/checkout@v2
       - uses: dart-lang/setup-dart@v1