Track read state of beta announcements in user profile

### Your use case

#### What would you like to do?
How nice would it be not needing to dismiss the "threads approaching beta" notif on each device?
#### Why would you like to do it?
i don't expect it is too bad for most people, but i got annoyed after the 5th or so since i have relatively many
#### How would you like to achieve it?
you could consider adding betas to `im.vector.web.settings` (or `im.vector.web.betas`) in users profiles in the future.

### Have you considered any alternatives?

_No response_

### Additional context

https://matrix.to/#/!zzGPdcyFqxIOJEijoI:matrix.org/$JmCa_iBKkXe6dzI2Vm_qJr3iceRXQoUo1NfR0_DxVXk?via=matrix.org&via=element.io&via=kde.org

No implementation files or tests are named. Start with the linked Matrix discussion and determine how beta-announcement read state should be represented in user profiles; done means dismissals persist across devices and the notification is not repeatedly shown.

Written by the indexing model from the issue text.