Simplified Sliding Sync

[dbkr]

Replaces the sliding sync option in labs with Simplified Sliding Sync so it will support the impl in Synapse directly rather than via the sliding sync proxy.

Requires https://github.com/matrix-org/matrix-js-sdk/pull/4400 Ported from https://github.com/element-hq/matrix-react-sdk/pull/36

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.
• [ ] I have licensed the changes to Element by completing the Contributor License Agreement (CLA)

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