How do I 'do-release-upgrade' now 21.10 is (just) out of support?

[Chris Green]

I'm sure I've not hit this problem before but when I try
'do-release-upgrade' on my [x]ubuntu 21.10 system it says:-

    root at t470# do-release-upgrade
    Checking for a new Ubuntu release
    Your Ubuntu release is not supported anymore.
    For upgrade information, please visit:
    http://www.ubuntu.com/releaseendoflife

    Please install all available updates for your release before upgrading.


It's only 4 months since 22.04 was released, it feels like there's
very little overlap between releases.

-- 
Chris Green