docs: add 0.21.0 migration guide
docs: add 0.21.0 migration guide

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dokku/dokku,progrium/dokku,dokku/dokku,dokku/dokku,claytonbrown/dokku,claytonbrown/dokku,dokku/dokku,progrium/dokku,dokku/dokku,dokku/dokku,claytonbrown/dokku,claytonbrown/dokku,progrium/dokku,progrium/dokku,claytonbrown/dokku,claytonbrown/dokku,claytonbrown/dokku,dokku/dokku,progrium/dokku,claytonbrown/dokku,dokku/dokku,progrium/dokku,progrium/dokku,progrium/dokku