Use calendar-end-users for End-User SIG invites

[danielgblanco]

This corrects the use of Google groups for the End-User SIG, to use [email protected] as the group to receive invites from:

1. SIG meetings (to discuss and coordinate SIG actions, which end-users may decide to join if interested, as any other SIG, but not focused on gathering user feedback or sharing learnings)
2. Recurring end-user monthly discussion sessions
3. Ad-hoc sessions for end-user feedback, panels, etc

It also drops WG from the name of the SIG.