Update rtbworkflow.yml to change .net versions

.github/workflows/rtbworkflow.yml

@@ -11,7 +11,6 @@ on:
       - 'com.adsbynimbus.nimbus/Runtime/Plugins/**'
       - '.github/workflows/rtbworkflow*'
 
-
 jobs:
   tests:
     name: Running CSharp Tests
@@ -22,6 +21,18 @@ jobs:
 
       - name: Setup dotnet
         uses: actions/setup-dotnet@v4
+        with:
+          dotnet-version: | 
+            3.1.x
+            5.0.x
+            6.0.x
+
+      - uses: actions/cache@v4
+        with:
+          path: ~/.nuget/packages
+          key: ${{ runner.os }}-nuget-${{ hashFiles('**/packages.config') }}
+          restore-keys: |
+            ${{ runner.os }}-nuget
 
       - name: Install dependencies
         run: nuget restore