feat(gc): add random jitter of 5 to 60 seconds to review due dates

Related to #732 (closed). Implemented as explained in #732 (comment 1042436138).

The default minimum sleep between GC worker runs is 5 seconds, thus I believe 5 is a sensible minimum value for the jitter. Although we can, I don't see the need to go higher than 60s for the maximum value. Additionally, the higher we go, the slower some related tests will become. We can update it again if we need to.