Use multiple .net versiosn

Author: Martin Boje Carpentier

.github/workflows/continous-benchmark.yml

@@ -17,6 +17,8 @@ jobs:
         uses: actions/setup-dotnet@v1
         with:
           dotnet-version: |
+            3.1.x
+            5.0.x
             6.0.x
       # Run benchmark with `go test -bench` and stores the output to a file
       - name: Show locations