Bump memory requirements to 3GB

Check https://data.firefox.com/dashboard/hardware to see how low the number of users with 2GB of RAM is. If it's still too high, postpone. Else, proceed.
Bump requirements in our doc
#5502

This used to block #15450 (closed) (!256 (comment 160642)), which is why lots of the comments are about this aspect of things, but that's not the case anymore.

Edited Jun 06, 2023 by intrigeri

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