Move deprecated things in `completions/*` to separate files

[scop]

Describe the feature/solution

Would be good to move deprecated things out of completions/* similarly as #870 does it for the main script ones.

We likely shouldn't just move them to 000_bash_completion_compat, because that one's loaded eagerly and the deprecation replacements are not loaded at that time. Unsure what would be the best way -- if we don't come up with a good enough one, I suppose we should just leave them where they are for the time being.

Maintenance (please complete the following information)

• This is a request for new completion
• Link to upstream project query about shipping the completion:

[scop]

I don't have a good solution for this in mind, so I'm removing this from the 2.12 release blocker list. We may close it altogether later unless some nice way is found.