Looser memory requirements when checking for upgrades

As documented on #5390 (closed), tails-upgrader-frontend-wrapper’s memory requirements are overly cautious. Without any action from our part, this would force us to implement #5502 in time for Tails 1.1, and be generally bad for users with “only” 1 GiB of RAM, who are already suffering a bit from the current state of things, even on Tails/Squeeze.

Feature Branch: bugfix/looser-memory-requirements-for-upgrades-check

Related issues

Related to #5502
Related to #5390 (closed)

Original created by @intrigeri on 7235 (Redmine)

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