How hot does the water need to be when cooking stock?
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:


The agitation from the bubbling at a full boil will emulsify and suspend more particles in the body o the stock, leading to a cloudier end product
The more rapid evaporation may require you to top off the stock pot with fresh water more often in order to keep the ingredients covered by the liquid