ci: don't install net core 2.1 (#120)

Author: phnx47-bot

.github/workflows/ci.yml

@@ -23,7 +23,6 @@ jobs:
         uses: actions/setup-dotnet@v4
         with:
           dotnet-version: |
-            2.1.x
             3.1.x
             5.0.x
             6.0.x
@@ -46,7 +45,6 @@ jobs:
         uses: actions/setup-dotnet@v4
         with:
           dotnet-version: |
-            2.1.x
             3.1.x
             5.0.x
             6.0.x