swap mono and .net core builds on Appveyor

Author: bodewig

appveyor.yml

@@ -1,11 +1,11 @@
 version: 2.9.2.{build}
 
 image:
+  - Ubuntu2004
   - Visual Studio 2013
   - Visual Studio 2017
   - Visual Studio 2022
   - Ubuntu
-  - Ubuntu2004
 
 skip_tags: true
 
@@ -28,7 +28,7 @@ for:
       only:
         - image: Visual Studio 2017
         - image: Visual Studio 2022
-        - image: Ubuntu2004
+        - image: Ubuntu
     before_build:
       - dotnet restore XMLUnit.NET.sln
       - dotnet --version
@@ -41,7 +41,7 @@ for:
 
   - matrix:
       only:
-        - image: Ubuntu
+        - image: Ubuntu2004
     before_build:
       - nuget restore XMLUnit.NET.NetFramework.sln
     build_script: