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>

prdoc/pr_1921.prdoc

@@ -0,0 +1,19 @@
+title: Fix para-scheduler migration
+
+doc:
+  - audience: Core Dev
+    description: |
+      Changing the `MigrateToV1` migration in the `ParachainScheduler` pallet to be truly idempotent. It is achieved by wrapping it in a `VersionedMigration`.
+
+migrations:
+  db: []
+
+  runtime:
+    - pallet: "ParachainScheduler"
+      description: Non-critical fixup for `MigrateToV1`.
+
+crates:
+  - name: "polkadot-runtime-parachains"
+    semver: patch
+
+host_functions: []