Drop group_id from CD rollout, deployment, and service tables

What does this MR do and why?

Drop the group_id column, its foreign key to namespaces, and its index from the remaining three CD tables:

• cd_rollouts
• cd_deployments
• cd_services

The application and GraphQL layers no longer read or write group_id, and the column was ignore_column'd in a prior milestone, so these post-deployment drops are safe.

The ignore_column :group_id entries are intentionally kept in the models here and removed in a follow-up next milestone, once these drops have deployed (the ignore must outlive the drop to avoid selecting a dropped column during the rolling deploy).

This completes the group_id removal for CD; the other four tables (cd_artifact_sources, cd_versions, cd_version_sets, cd_version_set_entries) are dropped in !253381 (merged).

Data deletion

• What's deleted: only the unused group_id column (plus its index and FK) on cd_rollouts, cd_deployments, cd_services. No rows are deleted.
• Recovery: the migrations are reversible, down re-adds the column, index, and FK. The column values are not recoverable, but group_id has been unused since the app/GraphQL stopped reading and writing it, so there is nothing functional to recover.
• Records affected: these are CD tables with the feature being developed behind the FF with a small number of rows on GitLab.com; the dropped column is unused.
• User experience impact: none, group_id is not read or written anywhere.

References

• https://gitlab.com/gitlab-org/gitlab/-/work_items/602934

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