Promote WEBGL_security_sensitive_resources from proposal to draft

Per the discussion on the public WebGL mailing list:
https://www.khronos.org/webgl/public-mailing-list/archives/1310/msg00031.html

I chose the number 23 for the extension, right after the highest number 22 for WEBGL_shared_resources. Please let me know if this is not the right approach.