ci: remove .netcore 3.1

Author: egil

.github/workflows/publish.yml

@@ -35,7 +35,6 @@ jobs:
       uses: actions/setup-dotnet@v3
       with:
         dotnet-version: |
-          3.1.x
           6.0.x
           8.0.x
 
@@ -84,7 +83,6 @@ jobs:
       uses: actions/setup-dotnet@v3
       with:
         dotnet-version: |
-          3.1.x
           6.0.x
           8.0.x