Move CA1845 analyzer back to warning level (#33444)

* Move CA1845 analyzer back to warning level
* Fix warnings in src/ProjectTemplates/Shared/AspNetProcess.cs

.editorconfig

@@ -147,7 +147,7 @@ dotnet_diagnostic.CA1840.severity = warning
 dotnet_diagnostic.CA1841.severity = warning
 
 # CA1845: Use span-based 'string.Concat'
-dotnet_diagnostic.CA1845.severity = suggestion
+dotnet_diagnostic.CA1845.severity = warning
 
 # CA1846: Prefer AsSpan over Substring
 dotnet_diagnostic.CA1846.severity = warning

src/ProjectTemplates/Shared/AspNetProcess.cs

@@ -181,9 +181,9 @@ namespace Templates.Test.Helpers
                     ListeningMessagePrefix, StringComparison.Ordinal) + ListeningMessagePrefix.Length);
 
                 output.WriteLine($"Detected that ASP.NET application is accepting connections on: {listeningUrlString}");
-                listeningUrlString = listeningUrlString.Substring(0, listeningUrlString.IndexOf(':')) +
-                    "://localhost" +
-                    listeningUrlString.Substring(listeningUrlString.LastIndexOf(':'));
+                listeningUrlString = string.Concat(listeningUrlString.AsSpan(0, listeningUrlString.IndexOf(':')),
+                    "://localhost",
+                    listeningUrlString.AsSpan(listeningUrlString.LastIndexOf(':')));
 
                 output.WriteLine("Sending requests to " + listeningUrlString);
                 return new Uri(listeningUrlString, UriKind.Absolute);