fix: Set GroupNameTimestamp on group promotion (#6729)

Otherwise if an invite link is generated and the group is renamed then before the promotion, the
joined member will have the group name from the invite link, not the new one.

src/tools.rs

@@ -179,7 +179,7 @@ pub(crate) fn gm2local_offset() -> i64 {
 
 /// Returns the current smeared timestamp,
 ///
-/// The returned timestamp MUST NOT be sent out.
+/// The returned timestamp MAY NOT be unique and MUST NOT go to "Date" header.
 pub(crate) fn smeared_time(context: &Context) -> i64 {
     let now = time();
     let ts = context.smeared_timestamp.current();