By appending `/webhook` to the public URL (becoming `/hookshot/webhooks/webhook`)
and by only stripping the `/hookshot/webhooks` prefix,
we're effectively following what newer Hookshot versions advise
(see https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1681).

This change appears to be backward-compatible (old webhook URLs like `/hookshot/webhooks/:hookId` still work),
until Hookshot behavior changes.