Revert "Add .NET 3.1"

This reverts commit 6d38e37.

Author: laurynas

.github/workflows/main.yml

@@ -8,7 +8,6 @@ jobs:
         dotnet-version:
           - '2.1'
           - '2.2'
-          - '3.1'
     name: .NET ${{ matrix.dotnet-version }} sample
     steps:
       - uses: actions/checkout@v3