Support slnx solution files

https://devblogs.microsoft.com/visualstudio/new-simpler-solution-file-format/

Author: mikeminutillo

src/NServiceBus.AcceptanceTesting/AcceptanceTestingTransport/AcceptanceTestingTransportInfrastructure.cs

@@ -25,7 +25,7 @@ static string FindSolutionRoot()
 
         while (true)
         {
-            if (Directory.EnumerateFiles(directory).Any(file => file.EndsWith(".sln")))
+            if (Directory.EnumerateFiles(directory).Any(file => Path.GetExtension(file) is ".sln" or ".slnx"))
             {
                 return directory;
             }

src/NServiceBus.Core/Transports/Learning/LearningTransportInfrastructure.cs

@@ -35,7 +35,7 @@ static string FindStoragePath()
         while (true)
         {
             // Finding a solution file takes precedence
-            if (Directory.EnumerateFiles(directory).Any(file => file.EndsWith(".sln")))
+            if (Directory.EnumerateFiles(directory).Any(file => Path.GetExtension(file) is ".sln" or ".slnx"))
             {
                 return Path.Combine(directory, DefaultLearningTransportDirectory);
             }