[xaprepare] Update dependencies on Ubuntu 20.04 (#4358)

Ubuntu 20.04 (focal fossa) no longer includes the `libx32ncurses6-dev`
package. Don't try to install when running on Ubuntu version 20 or
higher.

Author: grendello

build-tools/xaprepare/xaprepare/ConfigAndData/Dependencies/Linux.Ubuntu.cs

@@ -31,7 +31,8 @@ protected override void InitializeDependencies ()
 			if (UbuntuRelease.Major < 18 || (UbuntuRelease.Major == 18 && UbuntuRelease.Minor < 10))
 				Dependencies.AddRange (preCosmicPackages);
 			else {
-				Dependencies.AddRange (cosmicPackages);
+				if (UbuntuRelease.Major < 20)
+					Dependencies.AddRange (cosmicPackages);
 				if (UbuntuRelease.Major < 19)
 					Dependencies.AddRange (preDiscoPackages);
 			}