Remove old pre-join UTD logic

[uhoreg]

and add a playwright test for new pre-join UTD

The playwright test depends on the docker image for Synapse to include https://github.com/element-hq/synapse/pull/17104

Checklist

• [ ] Tests written for new code (and old code if feasible).
• [ ] New or updated public/exported symbols have accurate TSDoc documentation.
• [ ] Linter and other CI checks pass.
• [ ] Sign-off given on the changes (see CONTRIBUTING.md).