What's the minimum water temperature for making stock? (from either meat and vegetables, if they're different)
Traditionally, you want it at a very slow simmer, approximately 200 F, 94 C. This is essentially the same for all stocks. This is one case where very few people actually use a thermometer; the visual indication of slow occassional bubbles is a good sign you are the right temperature.
You could probably go lower, but without the visual indication, a thermometer would be required, or even a feedback loup such as is present in a sous-vide setup or immersion circulator. In that case, anything above 140 F / 65 C should be safe, and reasonable extraction amd gelatin conversion times are likely starting around 180 F / 83 C. The lower the temperature, the longer it will take to get a good result.
In the other direction, the main reasons not to go to a full boil are: