This PR changes the registration order of the `MessageQueue` pallet so
that it is registered last.

This is necessary so that the
[on_initialize](https://github.com/Snowfork/snowbridge/blob/df8d5da8/parachain/pallets/outbound-queue/src/lib.rs#L267)
hooks for Snowbridge can run before `MessageQueue` delivers messages
using its own `on_initialize`.

Generally, I think this is preferable regardless of Snowbridge's
particular requirements. Other pallets may want to do housekeeping
before MessageQueue starts delivering messages.

I'm hoping this PR, if accepted, can be included in the same release as
https://github.com/paritytech/polkadot-sdk/pull/1246. As otherwise,
changing the order of pallet registration is an ABI-breaking change.