[19.5] Revert the Orbit feature backports

What does this MR do and why?

Reverts all Orbit backports on 19-5-stable-ee that are not small bug fixes. One git revert -m 1 per stable MR, newest first. All reverts apply with no conflict.

Revert list: https://gitlab.com/gitlab-org/orbit/knowledge-graph/-/work_items/1412#note_4099821867

#	Reverted stable MR	Merge commit	Original MRs
1	!265285 (merged)	29c140b8ffa4	!264905 (merged), !265236 (merged), and the stable-only change that turns on orbit_promo_access_gate and orbit_trial by default
2	!265205 (merged)	bc49010a6d4f	!265126 (merged)
3	!265147 (merged)	e409fe3e3d1c	!264917 (merged), !264873 (merged), !264924 (merged), !264923 (merged), !265010 (merged)
4	!264911 (merged)	8a8d5ff7c450	!264743 (merged), !264891 (merged), !264757 (merged), !264857 (merged)
5	!264806 (merged)	b5765ef770b8	!264100 (merged), !264616 (merged)
6	!264801 (merged)	2b31e229e824	!264103 (merged), !264186 (merged), !264460 (merged), !264666 (merged), !264495 (merged), !264685 (merged), !264673 (merged)
7	!264269 (merged)	52f6a06ba48c	!256404 (merged)
8	!264270 (merged)	c96c928ee3b1	!262223 (merged)
9	!263724 (merged)	e4383f16d7a0	GA backport, 46 MRs

What stays, migrations, and verification

Stays (small bug fixes, already in 19.5.1, no GA code needed):

• !263383 (merged) (backport of !262140 (merged))
• !262739 (merged) (backport of !262112 (merged))
• !262751 (merged) (backport of !262513 (merged))

Migrations. The revert deletes 20261016070913_add_repository_storage_to_p_knowledge_graph_code_indexing_tasks and their db/schema_migrations files, and removes their changes from db/structure.sql. v19.5.2-ee already holds the reverted commits. If an instance upgraded to it, the tables stay in its database with no code that uses them. A later upgrade to 19.6 or later skips these migrations, because the versions are already recorded.

Check. On the files that the revert changes, the tree is the same as the stable branch before the GA backport. The only difference is on 19.5: one structure.sql line from a later non-Orbit backport (audit_events_instance_streaming_event_type_filters.kind). That line stays.

Undercoverage. pipeline:skip-undercoverage is set from the start: the revert deletes code and specs, so the job has no new code to cover.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
  • Not applicable: this MR reverts feature backports.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the E2E Omnibus GitLab EE pipeline (triggered by thee2e:test-on-omnibus-ee job) has succeeded. If it has failed (or shows warnings), investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.