Remove `check-migrations` for rococo chain (#6061)

This PR removes the `check-migrations` pipelines for all Rococo chains
because they are going down:
https://github.com/paritytech/devops/issues/3589, and these checks are
starting to fail, e.g.:

https://github.com/paritytech/polkadot-sdk/actions/runs/11339904745/job/31535485254?pr=4982

https://github.com/paritytech/polkadot-sdk/actions/runs/11339904745/job/31535486189?pr=4982

https://github.com/paritytech/polkadot-sdk/actions/runs/11339904745/job/31535486471?pr=4982

Additionally, `coretime-westend` was added to the `check-migrations`
matrix.