etherpad_base_url always ends with a trailing slash, and the playbook
passed it verbatim to jitsi_etherpad_base, which the Jitsi role appends
path segments to (e.g. /p/). The generated Jitsi configuration therefore
contained URLs like https://etherpad.example.com//p/, which can 404
depending on the web server. Strip trailing slashes when wiring.

Fixes #3471

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>