GitHub Actions: test Dart SDK 2.15, use latest for older.

Author: greenrobot-team

.github/workflows/dart.yml

@@ -42,8 +42,9 @@ jobs:
           - ubuntu-20.04
         sdk:
           - stable
-          - 2.14.0
-          - 2.13.0
+          - 2.15.1
+          - 2.14.4
+          - 2.13.4
           - 2.12.0  # currently the lowest fully supported version (i.e. generator + lib)
     runs-on: ${{ matrix.os }}
     steps: