feat: Don't depend on cleartext Chat-Version, In-Reply-To, and References headers for prefetch_should_download (#7932)

Don't depend on these 3 cleartext headers for the question whether we
download a message.

This PR will waste a bit of bandwidth for people who use the legacy
show_emails option; apart from that, there is no user-visible change
yet. It's a preparation for being able to remove these headers, in order
to further reduce unencrypted metadata.

Removing In-Reply-To and References will be easy; removing Chat-Version
must happen at least one release after the PR here is released, so that
people don't miss messages. Also, maybe some nerds depend on the
Chat-Version header for server-side filtering of messages, but we shall
have this discussion at some other time.

For the question whether a message should be moved, we do still depend
on them; this will be fixed with
https://github.com/chatmail/core/pull/7780.

When both this PR and #7780 are merged, we can stop requesting
Chat-Version header during prefetch.

src/imap/session.rs

@@ -21,7 +21,6 @@ const PREFETCH_FLAGS: &str = "(UID INTERNALDATE RFC822.SIZE BODY.PEEK[HEADER.FIE
                               DATE \
                               X-MICROSOFT-ORIGINAL-MESSAGE-ID \
                               FROM \
-                              IN-REPLY-TO REFERENCES \
                               CHAT-VERSION \
                               CHAT-IS-POST-MESSAGE \
                               AUTO-SUBMITTED \