backupninja: update livecheck

The existing `livecheck` block for `backupninja` checks the Git tags
for the 0xacab.org/liberate/backupninja repository. This works fine
but the Git repository `url` string is the same as the one that
livecheck generates from the `stable` URL, so we can simply use
`url :stable` instead.

Formula/b/backupninja.rb

@@ -6,7 +6,7 @@ class Backupninja < Formula
   license "GPL-2.0-or-later"
 
   livecheck do
-    url "https://0xacab.org/liberate/backupninja.git"
+    url :stable
     regex(/^backupninja[._-]v?(\d+(?:\.\d+)+)$/i)
   end