Remove stable from test matrix, replace with 2.19.6

Author: alanknight-wk

.github/workflows/dart_ci.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        sdk: [ 2.12.0, stable ]
+        sdk: [ 2.12.0, 2.19.6 ]
     steps:
       - uses: actions/checkout@v2
       - uses: dart-lang/[email protected]
@@ -46,7 +46,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        sdk: [ 2.12.0, stable ]
+        sdk: [ 2.12.0, 2.19.6 ]
     steps:
       - uses: actions/checkout@v2
       - uses: dart-lang/[email protected]