Relax `force_default_xcm_version` for testnet system parachains (#2385)

This PR fixes two things:
- relax `force_default_xcm_version` for testnet system parachains (e.g.
BridgeHubWestend has now 2 and there is no way to change it to 3, so we
need to call `force_xcm_version(3)` for every parachain that it is
connected to, because we send XCMv3 messages)
- add `Storage` item to `PolkadotXcm` pallet definition (now we cannot
see storage items for `pallet_xcm` in PJS)


## TODO

- [ ] when merged open PR to `polkadot-fellows/runtimes` repo