docs: remove coreos-migration

[tormath1]

It's time to move on.

[Copilot]

Pull request overview

This PR removes the CoreOS migration documentation section from the Flatcar website docs and eliminates remaining in-site references to that section.

Changes:

• Remove the coreos-migration documentation section (index + “update-from-container-linux” page).
• Remove references to CoreOS migration content from GCP deployment docs and update strategy docs.
• Remove the “CoreOS Migration” entry from the main docs landing page.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
content/docs/latest/updates-releases/releases/update-strategies.md	Removes the CoreOS migration link and replaces it with a brief “airgapped-like” note for outdated SSL certs.
content/docs/latest/deploy/cloud/gcp.md	Removes the “Upgrade from CoreOS Container Linux” section that linked to the migration docs.
content/docs/latest/coreos-migration/update-from-container-linux.md	Deletes the “Updating from CoreOS Container Linux” page (including its legacy aliases).
content/docs/latest/coreos-migration/_index.md	Deletes the CoreOS migration section index (including its legacy aliases).
content/docs/latest/_index.md	Removes the “CoreOS Migration” row from the main docs sections table.

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[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated no new comments.

Suppressed comments (2)

content/docs/latest/_index.md:26

• Removing the CoreOS migration section and pages will likely create 404s for existing site content that still links to the legacy URLs (e.g., content/blog/2020-03-18-update-from-coreos-cl-to-flatcar-cl.md links to https://docs.flatcar-linux.org/os/migrate-from-container-linux/ and /os/update-from-container-linux/). Consider keeping redirect stubs (aliases) for these legacy paths or updating the referring pages as part of this change to avoid broken links.

| [Diagnostics and Fixing Issues](./diagnostics/) | Troubleshooting tools, logs, and rollback guidance. |
| [Nebraska Update Manager & Releases](./updates-releases/) | Nebraska, release channels, and update management. |
| [Security](./security/) | Hardening, authentication, encryption, and supply-chain topics. |
| [Developer Guides](./devguide/) | SDK and image-building documentation for developers. |
| [How to Contribute](./contribute/) | Community and contribution information. |

content/docs/latest/updates-releases/releases/update-strategies.md:301

• The updated guidance recommends downloading the update payload over plain HTTP, but the “Airgapped updates” steps it references use HTTPS and already provide a safer workflow (download on another machine and copy over). As written, this can confuse readers and unnecessarily encourages an insecure transport.

Even if Flatcar tries to stay compatible with old versions, there are still some cases where it's not possible to update from an old release. The first common problem is that the SSL certificates might be so outdated that contacting the update server and downloading the update payload fails. In that case, download the update payload over plain HTTP (instead of HTTPS) and apply it with `flatcar-update` as described in the airgapped update steps above.

[t-lo]

LGTM - Great catch, thank you!