Hi @abest,
Earlier this year (in April) the original signing key for this repository was revoked because we were unable to prove that its private counterpart wasn’t exposed by a CircleCI security incident. If the last time this was working for you was during or before April then this might be the cause of the problem.
However, I can see that the first three lines of the public key block you shared match the public key I just fetched from the server, so this might not actually be the problem. I suggest fetching the current key (linked from the Official Packaging Guide you already found) and checking whether the whole thing matches what’s in your cloud-config YAML file.
Unfortunately at the moment I don’t have any other ideas to share. I’m not familiar with this specific technique, and the documentation for this apt cloud-config module didn’t give me any clues: what you’ve shared seems consistent with what’s mentioned in the documentation for sources, although the formatting of that part of the docs is currently a little hard to read. 