Report withheld messages to posthog using RoomKeysWithheldForUnverifiedDevice code

[richvdh]

There is a specific Posthog error code for reporting UTDs that are due to withheld messages: we should (make sure we) use it.

• Message key "withheld" reasons are not shown in UI nor reported to Posthog matrix-org/matrix-rust-sdk#4270

[uhoreg]

Looks like we use the right code in EW: https://github.com/element-hq/element-web/blob/83777a6c5788c204782c28bc0f225c4d53b4936b/src/DecryptionFailureTracker.ts#L76

[andybalaam]

Presumably this is just Element X then

[andybalaam]

This should only affect people receiving messages from someone who has turned on "only encrypt to verified devices" in Element Web.