remove old Dart 2.13 from CI

Author: robbecker-wf

.github/workflows/dart_ci.yaml

@@ -14,7 +14,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        sdk: [ 2.13.4, 2.18.7, stable, beta ]
+        sdk: [ 2.18.7, 2.19.6, stable ]
     steps:
       - uses: actions/checkout@v2
       - uses: dart-lang/setup-dart@v1