feat: adds unpublish release pipeline action

[tiagoapolo]

Thanks for submitting a PR! Please check the boxes below:

• I have added information to docs/ if required so people know about the feature!
• I have filled in the "Changes" section below?
• I have filled in the "How did you test this code" section below?
• I have used a Conventional Commit title for this Pull Request

Changes

#5799

How did you test this code?

use branch clone-unpublish-pipelines-be-fe

Case 1 - No In-flight feature

• create a release pipeline
• publish it
• unpublish it

Case 2 - In-flight feature

• create a release pipeline with a wait for condition for 1 day
• publish it
• add a feature
• unpublish action will be disabled and a tooltip informing that there's an feature in-flight

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
flagsmith-frontend-preview	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jul 22, 2025 3:20pm
flagsmith-frontend-staging	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jul 22, 2025 3:20pm

1 Skipped Deployment

Name	Status	Preview	Comments	Updated (UTC)
docs	⬜️ Ignored (Inspect)	Visit Preview		Jul 22, 2025 3:20pm

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-e2e:pr-5797	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api-test:pr-5797	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api:pr-5797	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-5797	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-5797	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-frontend:pr-5797	Finished ✅	Results ✅

[Zaimwa9]

One forgotten log to remove CreateReleasePipeline.tsx:119 existingPipeline undefined, a minor type change and good to go
(and rebase to include backend)