curve25519 used for end-to-end, but not for TLS

[nisbet-hubbard]

Is your feature request related to a problem? Please describe.
I get it that curve25519 is being used for end-to-end encryption, which is good until matrix-org/vodozemac#12 happens. But it’s not being used for TLS.

Describe the solution you'd like
So currently, this is what nginx says when it talks to the client:

prime256v1:secp384r1:secp521r1->prime256v1

If curve25519 was implemented for TLS, X25519 would appear instead.

Describe alternatives you've considered
Even when P256 is used for TLS, the end-to-end algorithm remains intact. So the alternative/status quo is to only use curve25519 in the end-to-end logic.

Additional context
https://spec.matrix.org/unstable/client-server-api/#end-to-end-encryption