Ignore sender keys in encrypted events

### Your use case

#### What would you like to do?

Update the web client to be compatible with the new Cross Domain Gateway (CDG) currently under development.

#### Why would you like to do it?

To support an ongoing public-sector project.

#### How would you like to achieve it?

For clients to work with the CDG, they need to stop using the event.content.sender_key param. It should only be being used to store/lookup the crypto sessions, where they can use the session ID instead.

### Have you considered any alternatives?

_No response_

### Additional context

The target delivery date for this project is the end of January, but there's some flexibility if that isn't possible.

Talk to @emmaburton1 for more context on the project schedule and priority.