[tests] fix InvalidTargetPlatformVersion for net8.0 (#9110)

Context: dotnet/sdk@25b360d

* Use `net8.0` `$(TargetFramework)`

When 60dbcc9 was backported to .NET 8, the accompanying unit test was accidentally targeting .NET 9.

* Don't test for `android33` *or* `android32`

`android33` is valid, so we *could* use `android32` to test an invalid number, however...

dotnet/sdk@25b360d was not backported to .NET 8, so invalid API levels that are older will not
give a `NETSDK1140` error.

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/BuildTest.cs

@@ -833,9 +833,9 @@ public void IfAndroidJarDoesNotExistThrowXA5207 ([Values(true, false)] bool buil
 		}
 
 		[Test]
-		public void InvalidTargetPlatformVersion ([Values ("android33", "android99.0")] string platformVersion)
+		public void InvalidTargetPlatformVersion ([Values ("android99.0")] string platformVersion)
 		{
-			const string targetFramework = "net9.0";
+			const string targetFramework = "net8.0";
 			var project = new XamarinAndroidApplicationProject {
 				TargetFramework = $"{targetFramework}-{platformVersion}",
 			};