Moving 11 May 2021 Silver Call to joint call with AGWG

As we did last week, we are going to move the Tuesday 11 May 2021 to 
have a joint call with the Accessibility Guidelines Working Group 
(AGWG).  WCAG3 topics will be for the first 60 minutes of the call.

Agenda
    - Acknowledgements section of the heartbeat [10 minutes]
    - Continue next steps and WCAG 3.0 Discussion [60-90 minutes]

We will be using the AGWG Zoom and IRC.

   * Meeting call-in and zoom information 
<https://www.w3.org/2017/08/telecon-info_ag>:

   * IRC: https://irc.w3.org <https://irc.w3.org>
     port: 6665 channel #ag