Some tests have a dependency on mono, which is deprecated and not part of ubuntu 24.04

[DaveTryon]

Our PR's have begun to fail in ubuntu-latest, with the following error:

System.IO.FileNotFoundException: Could not find 'mono' host. Make sure that 'mono' is installed on the machine and is available in PATH environment variable.
   at Microsoft.VisualStudio.TestPlatform.CrossPlatEngine.Helpers.DotnetHostHelper.GetMonoPath() in /_/src/Microsoft.TestPlatform.CoreUtilities/Helpers/DotnetHostHelper.cs:line 93
   at Microsoft.VisualStudio.TestPlatform.CrossPlatEngine.Hosting.DefaultTestHostManager.GetTestHostProcessStartInfo(IEnumerable`1 sources, IDictionary`2 environmentVariables, TestRunnerConnectionInfo connectionInfo) in /_/src/Microsoft.TestPlatform.TestHostProvider/Hosting/DefaultTestHostManager.cs:line 218
   at Microsoft.VisualStudio.TestPlatform.CrossPlatEngine.Client.ProxyOperationManager.SetupChannel(IEnumerable`1 sources, String runSettings) in /_/src/Microsoft.TestPlatform.CrossPlatEngine/Client/ProxyOperationManager.cs:line 226
   at Microsoft.VisualStudio.TestPlatform.CrossPlatEngine.Client.ProxyExecutionManager.InitializeTestRun(TestRunCriteria testRunCriteria, IInternalTestRunEventsHandler eventHandler) in /_/src/Microsoft.TestPlatform.CrossPlatEngine/Client/ProxyExecutionManager.cs:line 182

Based on actions/runner-images#10636. ubuntu-latest has updated to ubuntu-24.04, which does not include mono (mono has been deprecated). I'll open a PR to use ubuntu-22.04 in the runner to unblock the PR pipeline, but we need a plan to address this in the long run. Eventually, both PR pipeline and the CI pipeline should migrate to ubuntu-24.04.