Refresh draft WG charter in preparation for rechartering

[tidoust]

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. I put Q2 2025).

A link to the Conformance Test Suite was also added.

@Kangz @kdashg Let me know if you spot anything that you were not expecting here. If that looks ok-ish, I'll merge and start asking people to review the resulting draft. The group is obviously welcome to suggest changes as well!

[tidoust]

I'm taking the liberty to merge the pull request now. That's not meant to signal that the GPU for the Web group agrees with the proposed draft, it's just that I need an actual published HTML document that I can point people at to initiate horizontal reviews, and a pull request does not give me that. You're all welcome to raise issues and/or pull requests against the proposed draft to suggest changes!