Backport 4229 Into v1.6.0 (#4288)

Backport of #4229 into the v1.7.0 release in order to support dependent
runtimes. This version of the SDK doesn't have
`UncheckedOnRuntimeUpgrade`, so I do it the "old way".

Author: EgorPopelyaev

Cargo.lock

@@ -17,7 +17,7 @@
 //! A module that is responsible for migration of storage for Collator Selection.
 
 use super::*;
-use frame_support::traits::{OnRuntimeUpgrade, UncheckedOnRuntimeUpgrade};
+use frame_support::traits::OnRuntimeUpgrade;
 use log;
 
 /// Migrate to v2. Should have been part of <https://github.com/paritytech/polkadot-sdk/pull/1340>.