MM-57152: Get webconn count from the whole cluster (#26813)

We were setting the user status to offline without
checking for connections on other nodes in a cluster.

Now we implement a request-response mechanism for the whole
cluster and we check that before setting a user to offline.

https://mattermost.atlassian.net/browse/MM-57153

```release-note
Fix a bug where the user status would incorrectly be set to offline
without checking for connections in other nodes in an HA cluster.
```

Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>
Co-authored-by: Mattermost Build <build@mattermost.com>
Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>

server/channels/app/platform/web_conn.go

@@ -372,17 +372,6 @@ func (wc *WebConn) isSet(val string) bool {
 	return val != UnsetPresenceIndicator
 }
 
-// areAllInactive returns whether all of the connections
-// are inactive or not.
-func areAllInactive(conns []*WebConn) bool {
-	for _, conn := range conns {
-		if conn.active.Load() {
-			return false
-		}
-	}
-	return true
-}
-
 // GetSession returns the session of the connection.
 func (wc *WebConn) GetSession() *model.Session {
 	return wc.session.Load()