(build) Add .NET 3.1.x as well

This is currently still needed by the Test projects.  This will need to
be reviewed before another release is completed.

Author: gep13

.github/workflows/build.yml

@@ -26,10 +26,12 @@ jobs:
       - name: Fetch all tags and branches
         run: git fetch --prune --unshallow
 
-      - name: Install .NET SDK 5.0.x
+      - name: Install .NET SDK 3.1.x and 5.0.x
         uses: actions/setup-dotnet@v3
         with:
-          dotnet-version: '5.0.x'
+          dotnet-version: |
+            '3.1.x'
+            '5.0.x'
 
       - name: Cache Tools
         uses: actions/cache@v2