Switch cd_* tables to organization_id sharding key

What does this MR do and why?

MR 1 of 4 to implement changes requested in https://gitlab.com/gitlab-org/gitlab/-/work_items/602442

Introduces organization_id as the sharding key on the existing Continuous Deployment tables. This is the first, purely additive step of aligning the cd_* schema with the Rails design — no columns are dropped here.

• organization_id becomes the sharding key
• cd_applications / cd_environments: organization_id becomes the required sharding key (the XOR num_nonnulls(group_id, organization_id) = 1 check is dropped, organization_id gets a NOT NULL check). group_id stays as an optional owner — not touched.
• cd_services, cd_versions, cd_version_sets, cd_version_set_entries, cd_artifact_sources, cd_rollouts, cd_deployments: add organization_id (bigint) + concurrent index + FK → organizations (ON DELETE CASCADE)

• NOT NULL constraint.

group_id demoted, not dropped

On the 7 swap tables, group_id is:

• made nullable (change_column_null … true), and
• ignore_column'd in the model

References

https://gitlab.com/gitlab-org/gitlab/-/work_items/602442

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