Switch to stable endpoint/fields for MSC2858

[SBiOSoftWhare]

[reivilibre]

Element iOS is still using the unstable MSC2858 endpoint (usage here, definition here).

We're looking at getting rid of the unstable endpoints from Synapse soon as there are stable endpoints now (see matrix-org/synapse#10103), but we're blocked on this.

[richvdh]

there are stable endpoints now (see matrix-org/synapse#10103),

to be precise, there have been stable endpoints since March (matrix-org/synapse#9617) :).

[grantm]

Sorry typed this comment in the wrong thread