Update yubikey documentation with how to renew expired GPG keys.
The current method of installing yubikeys is documented here -- https://gitlab.com/gitlab-com/runbooks/-/blob/master/docs/uncategorized/yubikey.md.
A number of us however, used this older method -- 55404a63 -- which involved creating a mount point, using that as the GNUPGHOME to save secret keys, and then creating keys from there.
For those who have used that method and find themselves with expired keys, there's no current documentation on how to update them.
I will fix that.