Contracts: Use RuntimeUpgrade hooks instead of Hooks::on_runtime_upgrade (#2570)

* Fixes

* Remove on_runtime_upgrade hook

* remove upgrade_fn / add doc to Migration struct

* Add cumulus_pallet_*::migration to Migrations type

* add docstring

* fix

* Update parachain-template/runtime/src/lib.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Update parachains/runtimes/bridge-hubs/bridge-hub-rococo/src/lib.rs

* Update pallets/parachain-system/src/migration.rs

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Co-authored-by: Bastian Köcher <git@kchr.de>