Revert to installing tor from torproject's buster suite

If #16687 (closed) is merged with 585ca1efd1d6e4c1398d6d4b81016716ac11ad7d essentially as-is, we should revert most of that commit (modulo solving conflict correctly wrt. the APT snapshot bump) once the torproject’s stretch suite has tor 0.4.x.

Feature Branch: bugfix/16790-buster-revert-tor-installed-from-experimental

Related issues

Related to #16883 (closed)
Blocked by #16687 (closed)
Blocks #16209

Original created by @intrigeri on 16790 (Redmine)

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