Refresh draft WG charter in preparation for rechartering (#22)

Main changes:
- The boilerplate text was aligned with the charter template. Mostly editorial
refreshes and clarifications in the Success criteria section.
- The working mode was switched to "no intent to advanced to REC" mode.
- Completion dates updated (note "completion" means time when specs get
published as a CR Snapshot in the new mode, that's hopefully soon-ish).

A link to the Conformance Test Suite was also added.