Enable async backing on kusama

This referendum is meant for enabling async-backing on kusama, more information about what is async backing and what this parameter changes will do can be found here async backing

Currently on kusama, all async backing logic has been enabled since v1.1.0 runtime was enacted, but it is running in sync mode, parachains can produce block only at 12s rate.

After this referendum parachains can decide to produce blocks at 6s rate, by following this guide https://wiki.polkadot.network/docs/maintain-guides-async-backing, but that is not mandatory, they will still run at 12s rate without any changes needed on their side.

Testing done

Async backing has been running on Rococo since December 2023 and on Westend since February 2023 with all system parachains migrated to produce blocks at 6s rate.

Our final checklist enablement can be found here: https://github.com/paritytech/polkadot-sdk/issues/3226

Dependencies

Some critical fixes for async backing are in the upcoming v1.2 runtime release, so this referendum is configured to be enacted AFTER v1.2 https://kusama.subsquare.io/referenda/373 release has been enacted.

Whitelisted by: https://kusama.subsquare.io/fellowship/referenda/70

Known issues

No critical issues are known at this moment, however since the async backing subsystems in sync mode has been enabled a few issues that have arosed, this referendum won't change anything regarding that.

• There are still around 3-5% validators in the active set that are not upgrade, so when it is their turn to
  back they won't back anything, and if 2 out of 3 nodes are in the same backing group the group won't be
  able to back the candidates it is assigned to. Action Un-upgraded validators please upgrade to
  something higher than 1.8 node version.
• New validators entering the active set will get 0 backing point the first session they enter, Action No action needed it has been fixed with and will be included in 1.2 runtime release.
• Validators were accidentally changing they network-key(PeerId) at restart/upgradge which made them to not be properly discovered by the network for 36h and get 0 backing rewards. Action Double-check your validators doesn't do that, the network key is persistent next to the db so make sure it gets persisted across restarts, more details