housekeeping: Remove old frameworks from build

Author: glennawatson

.github/workflows/release.yml

@@ -22,16 +22,6 @@ jobs:
         fetch-depth: 0
         lfs: true
 
-    - name: Install .NET Core
-      uses: actions/setup-dotnet@v2.1.0
-      with:
-        dotnet-version: 3.1.x
-
-    - name: Install .NET 5
-      uses: actions/setup-dotnet@v2.1.0
-      with:
-        dotnet-version: 5.0.x
-
     - name: NBGV
       id: nbgv
       uses: dotnet/nbgv@master