[release/9.0.1xx] pass -Xlint:-options for javac -target/source 1.8

Fixes: #9925
Partial backport of: #10050

For backporting to .NET 9, we just need to suppress a warning that JDK
21 will show when using `javac -source 1.8 -target 1.8`.

Author: jonathanpeppers

src/Xamarin.Android.Build.Tasks/Tasks/Javac.cs

@@ -71,6 +71,10 @@ protected override string GenerateCommandLineCommands ()
 			} else {
 				cmd.AppendSwitchIfNotNull ("-target ", JavacTargetVersion);
 				cmd.AppendSwitchIfNotNull ("-source ", JavacSourceVersion);
+				// Ignore warning when targeting older Java versions
+				// JAVAC : warning : [options] source value 8 is obsolete and will be removed in a future release
+				// JAVAC : warning : [options] target value 8 is obsolete and will be removed in a future release
+				cmd.AppendSwitchIfNotNull ("-Xlint:", "-options");
 			}
 
 			return cmd.ToString ();