chore(messenger): update E2EE sheet and footer copy

[bmc08gt]

Updates the E2EE learn-more sheets and the group profile footer to the revised copy in the E2EE Messaging design (nodes 10416:1533, 10557:1412, 10557:1103).

• Group footer: "Group chats aren't end-to-end encrypted" is now "Public groups are not end-to-end encrypted".
• Group sheet: the title is "Public groups are not encrypted" and the body says messages are stored "in plain text". The lists are "Messages and media in 1:1 chats" (encrypted) and "Public group chats, including this one" plus the payments line (not encrypted).
• DM sheet: the body says "media" instead of "photos" and drops "Not even Flipcash can." Encrypted is "All message text" and "All media". Not encrypted drops "That a message was edited or deleted", renames "Group chats" to "Public group chats", and ends with "Messages sent before encryption became available".
• Payments read "Payments, which are recorded on the Solana blockchain" on both sheets.

Two typos in the design are fixed here: "Public groups chats" and "Messages in public group are stored".

The item_e2eeSheet_* keys are renamed to match their new text, the payments line is one shared string, and the strings no longer used are removed.

iOS counterpart: code-payments/code-ios-app#943.