security/acme-client: add button to update SFTP/SSH host keys

[fraenki]

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Is your feature request related to a problem? Please describe.
Currently the SFTP automation does not offer a way to update the SFTP/SSH host key. When the host key changes, the SFTP automations fails to connect to the target host.

Describe the solution you'd like
A new button should be added to purge or update the host key. Maybe Purge Host Keys...

Describe alternatives you've considered
It is possible to manually purge an outdated host key from /var/etc/acme-client/sftp-config/known_hosts, but that's cumbersome and error-prone.

Additional context
none