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This reverts commit d1a2c9dc.

Merge remote-tracking branch 'spriver/de_persistent_passphrase' into review-de_persistent_passphrase

* The use of ${RELEASE_BRANCH} worked by coincidence since it can be
  either 'stable' or 'testing', and there happens to also exist Debian
  code names like that, and we set the same expiry for all of
  them. Let's fix on 'stable' which always should exist.
* The old url was only pointing to the 'debian' archive, and never
  e.g. the 'torproject' one.

Refs: #12171

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Update recommended memory size for VirtualBox and virt-manager to match our current system requirements.