WebSocket connections fail on Tomcat 9

STEPS TO REPRODUCE

1. Start an XWiki instance with Tomcat 9 (tested with 9.0.54)
2. Install XWiki Platform - Realtime - Wiki - UI application (or/and XWiki Platform - Realtime - WYSIWYG - UI)
3. If the case, go to 'XWikiPreferences' page, edit it in object mode and add an object of type 'XWiki.EditorBindingClass' and set (for XWiki Platform - Realtime - Wiki - UI):

Data Type: 'org.xwiki.rendering.syntax.SyntaxContent#text'
Role Hint: 'realtime-wiki'

      4. Edit a page in Wiki mode and observe the JS console

EXPECTED RESULTS

The user is connected to the collaborative session.

ACTUAL RESULTS

The connection to the collaborative session doesn't succeed and in browser's JS console there are some error messages (see screenshots attached):

WebSocket connection to 'ws://localhost:1115/xwiki/websocket/xwiki/netflux' failed: 

In XWiki console, when the server starts, there is the following warning displayed:

2021-10-21 15:36:29,943 [main] WARN x.w.i.XWikiEndpointInitializer - The servlet container doesn't support the Java API for WebSocket (JSR 356).

The issue does not reproduce on Jetty.