1.6.11

Add new captcha type: hold button
Add cache compatibility layer: no-cache headers, DONOTCACHEPAGE, MU-phase cookie check
Add Vary: Cookie header option (Settings → Cookies → Cache Compatibility)
Add cache plugin incompatibility detection and admin alerts
Add CACHE-COMPATIBILITY.md with Nginx, Varnish, Apache, Cloudflare config examples
Add new security rules to block emerging threats
Import data security improvements
Update libraries and dependencies
Improve translation files
Fix minor bugs