Fix para-scheduler migration on Rococo (#1921)

Closes https://github.com/paritytech/polkadot-sdk/issues/1916

Changes:
- Trivially wrap the migration into a version migration to enforce
idempotency.
- Opinionated logging nits

@liamaharon maybe we can add a check to the `try-runtime-cli` that
migrations are idempotent? It should be possible to check that the
storage root is identical after executing a second time (and that it
does not panic like it did here 😆).

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>
Co-authored-by: Francisco Aguirre <franciscoaguirreperez@gmail.com>

substrate/frame/support/src/migrations.rs

@@ -119,7 +119,7 @@ impl<
 		let on_chain_version = Pallet::on_chain_storage_version();
 		if on_chain_version == FROM {
 			log::info!(
-				"Running {} VersionedOnRuntimeUpgrade: version {:?} to {:?}.",
+				"🚚 Pallet {:?} migrating storage version from {:?} to {:?}.",
 				Pallet::name(),
 				FROM,
 				TO
@@ -134,9 +134,10 @@ impl<
 			weight.saturating_add(DbWeight::get().reads_writes(1, 1))
 		} else {
 			log::warn!(
-				"{} VersionedOnRuntimeUpgrade for version {:?} skipped because current on-chain version is {:?}.",
+				"🚚 Pallet {:?} migration {}->{} can be removed; on-chain is already at {:?}.",
 				Pallet::name(),
 				FROM,
+				TO,
 				on_chain_version
 			);
 			DbWeight::get().reads(1)