postgres: Add Commvault backup compatibility

This slightly changes the default of the role by always installing postgresql-{{ postgres_version }} (and providing a sensible default for postgres_version), but this is usually the correct thing to do in Debian and a release upgrade requires manual work anyway.

postgres/defaults/main.yml

@@ -2,3 +2,7 @@
 
 postgres_pgdg_repo: false
 postgres_rsnapshot: false
+postgres_commvault_compat: false
+
+### Will be the version of the `postgresql` package if undefined
+# postgres_version: 15