tasks/sync-ssh-hostkeys: quiet ssh-keyscan banners

We don't want these comments to be added to docs/ssh-known_hosts.txt.

From OpenSSH 9.8 release notes [1]:

 * ssh-keyscan(1): this tool previously emitted comment lines
   containing the hostname and SSH protocol banner to standard error.
   This release now emits them to standard output, but adds a new
   "-q" flag to silence them altogether.

[1] https://www.openssh.com/txt/release-9.8