(build) add sdk to 6.0 for docs workflow

Author: arturcic

.github/workflows/docs.yml

@@ -128,6 +128,11 @@ jobs:
       with:
         path: tools
         key: tools-${{ runner.os }}-${{ hashFiles('./build/**') }}
+    -
+      name: Install .NET SDK 2.1.x
+      uses: actions/setup-dotnet@v1
+      with:
+        dotnet-version: '2.1.x'
     -
       name: Install .NET SDK 6.0.x
       uses: actions/setup-dotnet@v1