Enable mainnet system parachains to use async backing-enabled collator (#3630)

This is phase 2 of async backing enablement for the system parachains on
the production networks. ~~It should be merged after
polkadot-fellows/runtimes#228 is enacted. After it is released,~~ all
the system parachain collators should be upgraded, and then we can
proceed with phase 3, which will enable async backing in the runtimes.

UPDATE: Indeed, we don't need to wait for the runtime upgrade enactions.
The lookahead collator handles the transition by itself, so we can
upgrade ASAP.

## Scope of changes

Here, we eliminate the dichotomy of having "generic Aura collators" for
the production system parachains and "lookahead Aura collators" for the
testnet system parachains. Now, all the collators are started as
lookahead ones, preserving the logic of transferring from the shell node
to Aura-enabled collators for the asset hubs. So, indeed, it simplifies
the parachain service logic, which cannot but rejoice.