chore: add schema for juju 3.1.6

Author: james-garner-canonical

juju/client/SCHEMAS.md

@@ -8,6 +8,7 @@ Please see docs/CONTRIBUTING.md for the process for updating this file.
 3.1.3 (identical to 3.1.1)
 3.1.4 (identical to 3.1.1)
 3.1.5 (identical to 3.1.1)
+3.1.6
 
 # 3.3
 3.3.0