History sharing: Update warning when toggling on E2EE

When turning on E2EE for an existing room, a warning is shared. On EX, the warning mentions that history is visible only to members since invited or joined. As a minimum, we should eliminate or modify this part, as it will no longer be true. However, given there are discrepancies between these warnings on EX and EW we should make them consistent.

The current state is that **EX always shows one single warning**, regardless of the room's join rules or anything else:

"_Once enabled, encryption for a room cannot be disabled, Message history will only be visible for room members since they were invited or since they joined the room.
No one besides the room members will be able to read messages. This may prevent bots and bridges to work correctly.
We do not recommend enabling encryption for rooms that anyone can find and join._"

**EW shows a differentiates** depending on (at least) join rule. For `public`:

"_It's not recommended to add encryption to public rooms. Anyone can find and join public rooms, so anyone can read messages in them. You'll get none of the benefits of encryption, and you won't be able to turn it off later. Encrypting messages in a public room will make receiving and sending messages slower._

_To avoid these issues, create a new encrypted room for the conversation you plan to have._"

For others:

"_Once enabled, encryption for a room cannot be disabled. Messages sent in an encrypted room cannot be seen by the server, only by the participants of the room. Enabling encryption may prevent many bots and bridges from working correctly. Learn more about encryption._"

Start at the EX and EW flows for toggling E2EE on an existing room, then compare the warning text for public and other join rules. Confirm how each client selects its warning before deciding the consistent wording. Done means the outdated history statement is removed or corrected and the EX and EW warnings agree for the relevant room types.

Written by the indexing model from the issue text.