Update meeting times in README

[AetherUnbound]

Description

Small PR to update the meeting time for our weekly dev chat in our README (as announced here).

I also updated the Make WP "Slack" to "Chat" to make it more agnostic to the tool, and added more information on the monthly prioritization meeting.

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[dhruvkb]

I have a nit just to match the format of the "Monthly Prioritisation Meeting" time specification. Nothing worth blocking over, LGTM!