docs: deprecate protection-broken and related stuff (#7018)

came over these parts while targeting the new info message of
https://github.com/chatmail/core/pull/7008 in
https://github.com/deltachat/deltachat-ios/pull/2778 and
https://github.com/deltachat/deltachat-android/pull/3822

---------

Co-authored-by: Hocuri <hocuri@gmx.de>

deltachat-jsonrpc/src/api/types/chat.rs

@@ -70,7 +70,7 @@ pub struct FullChat {
     fresh_message_counter: usize,
     // is_group - please check over chat.type in frontend instead
     is_contact_request: bool,
-    is_protection_broken: bool,
+    is_protection_broken: bool, // deprecated 2025-07
     is_device_chat: bool,
     self_in_group: bool,
     is_muted: bool,