TEL migration guide: state that the move to Talos Enterprise Linux is one-way

Fast follow to docs#757, which is being merged as part of the Talos Enterprise Linux launch section. Part of #649.

The migration how-to (tel/how-to-guides/migrate-clusters-to-tel) tells a customer how to move a cluster onto Talos Enterprise Linux, but does not tell them the move cannot be undone. That is the one thing a reader most needs before they trigger the upgrade, so it should be stated on the page rather than learned afterwards.

What to add

A short note in the migration guide saying the move is one-way, placed before the steps rather than after them.

The mechanics, from omni#2760:

• Talos Enterprise Linux exists only for Talos v1.13.0 and later.
• Omni is connected to both factories and prefers the enterprise one for any version the two have in common. A user cannot choose the open-source factory for a version that Enterprise Image Factory also serves.
• Downgrading from Talos Enterprise Linux back to open-source Talos is prohibited.

So for a cluster already on v1.13.0 or later, the next upgrade of any kind moves it to enterprise images and there is no supported route back.

Also worth considering for the same page

The same issue records a second consequence that is arguably sharper for the reader: if Omni loses its connection to the factory a machine's images came from, it treats those machines as orphaned and will not upgrade them. The only permitted operation is to destroy them. Whether that belongs in the migration guide or somewhere in the enterprise factory concept pages is a judgement call, but it is currently written down nowhere a customer will see it.

Why it was not in the original PR

The point surfaced during launch-day discussion in #proj-omni on September 15, after the PR had been reviewed and approved. Splitting it out rather than holding the merge.

Open the migration how-to at tel/how-to-guides/migrate-clusters-to-tel and read the introductory content before the migration steps. Add a short warning there that moving to Talos Enterprise Linux is one-way and cannot be reverted to open-source Talos; the page is done when readers see this before starting the upgrade.