[Epic] Exclude Insecure Devices - Awareness & Rollout to All

[mxandreas]

Problem
As part of the invisible crypto initiative, devices which are insecure (aka unverified) will be excluded from sending & receiving messages. The corresponding work has been done, but is currently behind a feature toggle in the apps (Exclude insecure devices when sending/receiving messages). This is because it is a disruptive change to users who have not verified (all of) their devices.

Objective
The objective of this epic is to be able to turn that feature toggle ON in all Element clients with as little friction and surprises for users as possible. This mainly involves awareness activities (such as relevant blogposts) and banners/toasts in the apps which either inform users to verify their device or to set up recovery (to be ready to verify any new devices in the future).

The provisional date for excluding insecure devices is Oct 1st, 2025.

Issues in other organisations

• Incoming verification request is ignored if it arrives before the device list is updated matrix-org/matrix-rust-sdk#5007
• Megolm session retrieved from backup incorrectly marked as insecure matrix-org/matrix-rust-sdk#5018 -- a blocker for rollout