fix: bump contentful-migration from 4.20.2 to 4.21.0 [SPA-2047]

[Chaoste]

Ship the fix for experiences that was introduced in https://github.com/contentful/contentful-migration/pull/1320

[t-col]

changed the title so semantic-release will actually release a new version

[contentful-automation[bot]]

:tada: This PR is included in version 3.3.3 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: