Remove all usages of `relay_dispatch_queue_size`

[gavofyork]

Currently, a well-known key on the Relay-chain, relay_dispatch_queue_size, is used by parachains/cumulus to determine what messages (how many, how big) may be pushed up through UMP. This is a bit silly as it needs knowledge of the configuration items and certain assumptions on how the chain operations before it is useful.

Instead, there is now a new well-known key relay_dispatch_queue_remaining_capacity which retains the available amount of space and slots for UMP messages. All usages of the former should be removed in favour of this. This should lead to some simplification.

Relies on paritytech/polkadot#6271 .

[tifecool]

Down to carry this out as soon as paritytech/polkadot#6271 is complete.

[daredevil3435]

Hey, I want to strt contributing to polkadot.What are the prerequisites for that?

[bkchr]

@daredevil3435 as you see the prerequisity is not done yet.

[daredevil3435]

@bkchr oh okay gotcha

[bkchr]

The migration to relay_dispatch_queue_remaining_capacity will not be that easy. We first need to enact paritytech/polkadot#6271 on chain to have the new key available. Then we will need to wait for all parachains to have migrated to relay_dispatch_queue_remaining_capacity before we can remove relay_dispatch_queue_size.

With paritytech/cumulus#2608 Parachains will first try to load relay_dispatch_queue_remaining_capacity from the relay chain proof, before they fall back to relay_dispatch_queue_size. However, this will require runtime upgrades on the Parachains the storage proof read inside the runtime.