[winswitch] public keys

[Mark Couteau]

Hello List:

I keep having a failure to update from the xpra software server due to a 
public key issue.

I'm not very familiar with this sort of thing.

Here is the error message from the apt update command on Linux Mint 21 
Vanessa.

Hit:9 https://xpra.org jammy InRelease
Err:9 https://xpra.org jammy InRelease
   The following signatures couldn't be verified because the public key 
is not available: NO_PUBKEY 18ADB31CF18AD6BB
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
All packages are up to date.
W: An error occurred during the signature verification. The repository 
is not updated and the previous index files will be used. GPG error: 
https://xpra.org jammy InRelease: The following signatures couldn't be 
verified because the public key is not available: NO_PUBKEY 18ADB31CF18AD6BB
W: Failed to fetch https://xpra.org/dists/jammy/InRelease  The following 
signatures couldn't be verified because the public key is not available: 
NO_PUBKEY 18ADB31CF18AD6BB
W: Some index files failed to download. They have been ignored, or old 
ones used instead.

Any help is appreciated.