[physical][postgresql] concat→|| operator (#2945)

Use `||` standard concatenation instead of the `concat` function in
order to use the `vault_kv_store` index on `parent_path`.

physical/postgresql.go

@@ -72,7 +72,7 @@ func newPostgreSQLBackend(conf map[string]string, logger log.Logger) (Backend, e
 		delete_query: "DELETE FROM " + quoted_table + " WHERE path = $1 AND key = $2",
 		list_query: "SELECT key FROM " + quoted_table + " WHERE path = $1" +
 			"UNION SELECT DISTINCT substring(substr(path, length($1)+1) from '^.*?/') FROM " +
-			quoted_table + " WHERE parent_path LIKE concat($1, '%')",
+			quoted_table + " WHERE parent_path LIKE $1 || '%'",
 		logger: logger,
 	}