Can I pasteurize milk in a jam jar? The InstantPot manual says that I can use a bottle or container to make yogurt. It says to first pasteurize the milk (using the steam function to heat it to at least 180F/83C) then cool to 115F/46C and add starter. Can I use a jam jar for this?

I'm a bit afraid of breaking the glass - but on the other hand the jam jar should be prepared to handle some heat difference (namely between heated jam and room).
Any jar sold for home canning will work for what you want. They are designed to handle temperatures higher than 212°F/100°C.

What you want to avoid is thermal shock - a large and rapid change of temperature. Don't put cool/cold jar(s) in boiling water, don't put hot jar(s) directly on cool counter or in refrigerator/freezer. Don't fill cold jar(s) with hot contents and vice-versa.