Remove experimental support for MSC3903 v1 (#3184)

* v2 of MSC3903 implementation

This is a deliberate breaking change on an unstable feature.

* Reinstate v1 support to make this a non-breaking change

Deprecates several experimental types

* Remove MSC3903 v1 support

This is a breaking change in code marked unstable/experimental

Revert "Reinstate v1 support to make this a non-breaking change"

This reverts commit 8977345.

Author: hughns