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 😆

).

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Signed-off-by: Oliver Tale-Yazdi <[email protected]>
Co-authored-by: Liam Aharon <[email protected]>
Co-authored-by: Francisco Aguirre <[email protected]>