Preparatory steps for solo- and parachain migration

[haerdib]

The following preparatory steps need to be executed, before starting the migration:

• Make sure the sudo account has enough balance on the solo chain (balance migration!) or doesn't exist at all on the solochain side.
• Upgrade both chains to match each other. Including:
  • Use the same pallets -> Migration runtime upgrade and bump to polkadot v0.9.18 #83
  • Compatible substrate version -> bump to polkadot 0.9.18 #82
• Upgrade parachain runtime to filter extrinsic (do not allow any calls but runtime upgrades & sudo calls) -> Migration runtime upgrade and bump to polkadot v0.9.18 #83
• Prepare and test final solo chain runtime update to disable block production and disable all calls (Call Filter)
• Prepare and test parachain runtime update to add migration pallet -> Migration runtime upgrade and bump to polkadot v0.9.18 #83
• Benchmark new pallets
• Communicate downtime (exchange, users, ... )

All these steps need to be tested with a dry run (with the help of the test script developed in #79