fix(db): pg store connection called without self

### Summary

The PR #11480 introduced a bug that calls `store_connection`
without passing `self`. This fixes that.

Signed-off-by: Aapo Talvensaari <aapo.talvensaari@gmail.com>

kong/db/strategies/postgres/connector.lua

@@ -567,7 +567,7 @@ function _mt:query(sql, operation)
       -- we cannot cleanup the connection
       ngx.log(ngx.ERR, "failed to disconnect: ", err)
     end
-    self.store_connection(nil, operation)
+    self:store_connection(nil, operation)
 
   elseif is_new_conn then
     local keepalive_timeout = self:get_keepalive_timeout(operation)