templates/systemd/mastodon: update sandbox mode (#16235)

* templates/systemd/mastodon: add new sandboxing options

* templates/systemd/mastodon: add '@privileged' and remove duplicates SystemCallFilters

* templates/systemd/mastodon: add '@ipc' SystemCallFilter

* templates/systemd/mastodon: add '@memlock' SystemCallFilter

* templates/systemd/mastodon: allow '@resources' filter to mastodon-web service