Use 3.1.3 as stable SDK

Author: Ilya Yanok

.github/workflows/test-package.yml

@@ -24,7 +24,7 @@ jobs:
       - uses: actions/checkout@8ade135a41bc03ea155e62e844d188df1ea18608
       - uses: dart-lang/setup-dart@8a4b97ea2017cc079571daec46542f76189836b1
         with:
-          sdk: 2.19.0
+          sdk: 3.1.3
       - id: install
         name: Install dependencies
         run: dart pub get
@@ -37,7 +37,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        sdk: [2.19.0, dev]
+        sdk: [3.1.3, dev]
     steps:
       - uses: actions/checkout@8ade135a41bc03ea155e62e844d188df1ea18608
       - uses: dart-lang/setup-dart@8a4b97ea2017cc079571daec46542f76189836b1