Backport of "Update custom flows from beta to GA"

What does this MR do and why?

Backport of !239777 (merged) to the 19-2-stable-ee branch.

Custom flows moved from beta to generally available in 19.2. The original MR (!239777 (merged)) landed on master but missed the stable branch.

This backport cherry-picks that change onto 19-2-stable-ee. It covers:

• Removing the beta badge from custom flows across all AI Catalog pages (create, edit, duplicate, show)
• Updating the ai_catalog_flows maturity from :beta to :ga in the backend features catalogue
• Dropping Status: Beta from flow docs and adding GA version history entries
• Removing the now-unnecessary beta badge test assertions

How to set up and validate locally

Steps to verify the beta-to-GA change

1. On a Self-Managed instance, go to Admin > Settings > GitLab Duo > Change configuration.
2. Turn off Use experiment and beta GitLab Duo features and save.
3. Navigate to AI > Flows at the project level. Verify custom flows are still visible and no Beta badge appears.
4. Repeat at the group level (AI > Flows) and at Explore > AI Catalog > Flows.
5. Open the show, create, edit, and duplicate pages for a flow. Confirm no beta badge on any of them.

Backend check (rails console):

catalogue = Gitlab::Llm::Utils::AiFeaturesCatalogue::LIST
catalogue[:ai_catalog_flows][:maturity]
# => :ga

Specs:

bin/rspec ee/spec/lib/gitlab/llm/utils/ai_features_catalogue_spec.rb
yarn jest ee/spec/frontend/ai/catalog/pages/ai_catalog_item_show_spec.js
yarn jest ee/spec/frontend/ai/catalog/pages/ai_catalog_item_new_spec.js
yarn jest ee/spec/frontend/ai/catalog/pages/ai_catalog_item_edit_spec.js
yarn jest ee/spec/frontend/ai/catalog/pages/ai_catalog_item_duplicate_spec.js

See https://gitlab.com/gitlab-org/release/tasks/-/work_items/27481 for more details.

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.
• 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:test-on-omnibus-ee job has succeeded, or if it has failed, 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.