apt update fails for digitalocean apt repository "The repository is not updated and the previous index files will be used"

Using an Ubuntu 22 droplet. It has been working fine for a few months including regular apt updates. But today’s apt update has a failure.

$ apt update
Hit:1 https://repos.insights.digitalocean.com/apt/do-agent main InRelease
Hit:2 http://mirrors.digitalocean.com/ubuntu jammy InRelease
Hit:3 http://mirrors.digitalocean.com/ubuntu jammy-updates InRelease
Hit:4 http://mirrors.digitalocean.com/ubuntu jammy-backports InRelease
Get:5 https://repos-droplet.digitalocean.com/apt/droplet-agent main InRelease [5,518 B]
Hit:6 http://security.ubuntu.com/ubuntu jammy-security InRelease
Err:5 https://repos-droplet.digitalocean.com/apt/droplet-agent main InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 35696F43FC7DB4C2
Fetched 5,518 B in 7s (803 B/s)
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
7 packages can be upgraded. Run 'apt list --upgradable' to see them.
W: https://repos.insights.digitalocean.com/apt/do-agent/dists/main/InRelease: Key is stored in legacy trusted.gpg keyring (/etc/apt/trusted.gpg), see the DEPRECATION section in apt-key(8) for details.
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://repos-droplet.digitalocean.com/apt/droplet-agent main InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 35696F43FC7DB4C2
W: Failed to fetch https://repos-droplet.digitalocean.com/apt/droplet-agent/dists/main/InRelease  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 35696F43FC7DB4C2
W: Some index files failed to download. They have been ignored, or old ones used instead.

Does Digital Ocean have a known fix for this?

To be clear, apt is able to retrieve most updates, and I can still install packages using apt.