Bump gh minimum version

Avoid spurious failures around novel token format

Author: jennybc

DESCRIPTION

@@ -30,7 +30,7 @@ Imports:
     desc (>= 1.4.2),
     fs (>= 1.3.0),
     gert (>= 1.4.1),
-    gh (>= 1.2.1),
+    gh (>= 1.6.0),
     glue (>= 1.3.0),
     jsonlite,
     lifecycle (>= 1.0.0),