Use the new MatrixRTC backend/service discovery endpoint

[robintown]

Your use case

TODO: add more context. element-hq/synapse#18967 should at least have some of it.

Have you considered any alternatives?

No response

Additional context

No response

[RegiPab]

Do I need to add any additional data to help with this issue or everything provided in #3566 is enough?

[Half-Shot]

Completed in #3629