Restore `templates/debian-11.yaml`

[jandubois]

As you intended to keep templates/_images/debian-11.yaml, you might have removed templates/debian-11.yaml by accident?

Originally posted by @AkihiroSuda in #4029 (comment)

[jandubois]

We do keep templates for EOL versions of other distros, we just don't link to them from the README.