Bump memory requirements to 3 GB

• Check https://data.firefox.com/dashboard/hardware to see how low the number of users with 2 GB of RAM is. Also check bug reports sent with WhisperBack. If it's still too high, postpone. Else, proceed.
• Next time we bump RAM requirements: notify user... (#5502 - closed)
• ensure all Jenkins workers have enough RAM
• ensure all Jenkins workers have enough storage space wherever they store VM snapshots
  • Easiest way to do this may be to create a -force-all-tests branch that gives TailsToaster 3 GB of RAM, run the test suite on it, and monitor during the run what's the max disk usage.
• Bump requirements in our doc
• Give TailsToaster 3 GB of RAM
• Unlock potential follow-ups that were blocked by the 2 GB contraint:
  • This might help enable us to drop the 2-steps LUKS PBKDF memory cost dance if we wish to: the main reason we have this 2-steps dance is to reach our desired memory cost on 2 GB systems. There are other reasons though (#18040 (comment 265855)).
  • 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.