- update used .net sdk version

.github/workflows/ci.yml

@@ -9,7 +9,10 @@ jobs:
     
     steps:
     - name: Checkout
-      uses: actions/checkout@v1
+    - uses: actions/checkout@v1
+    - uses: actions/setup-dotnet@v1
+      with:
+        dotnet-version: '6.0.x'
     - name: Build
       run: dotnet build
   
@@ -20,7 +23,10 @@ jobs:
     
     steps:
     - name: Checkout
-      uses: actions/checkout@v1
+    - uses: actions/checkout@v1
+    - uses: actions/setup-dotnet@v1
+      with:
+        dotnet-version: '6.0.x'
     - name: Run tests
       run: dotnet test