Add .net 7

Author: MelbourneDeveloper

.github/workflows/buildandtest.yml

@@ -29,6 +29,11 @@ jobs:
         with:
           dotnet-version: 6.0.x
 
+      - name: Setup .NET Core SDK 7
+        uses: actions/setup-dotnet@v2
+        with:
+          dotnet-version: 7.0.x
+
       - name: Install dependencies
         run: dotnet restore src/RestClient.Net.sln