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

playbooks/tasks/sync-ssh-hostkeys.yml

@@ -16,7 +16,7 @@
     - name: Fetch known_hosts
       shell: |
         set -eo pipefail
-        ssh-keyscan -p {{ ansible_port | default(22) }} 127.0.0.1 2>/dev/null \
+        ssh-keyscan -q -p {{ ansible_port | default(22) }} 127.0.0.1 2>/dev/null \
         | sed -E 's/^(\[?)127\.0\.0\.1/\1{{ inventory_hostname }}/' \
         | sort
       environment: