I have not tried this and just explored config+code, but it looks like you can switch to a different Gravatar-compatible service using:
The tricky bit here being “Gravatar-compatible” which I don’t thing robohash is.
Robohash also accepts commands as paths, so https://robohash.org/nick?set=set3&bgset=bg1 is the same as https://robohash.org/set_set3/bgset_bg1/nick. This helps since you can encode the commands in the baseUrl. However, jitsi will append ?d=404&size=200 to the URL to specify size this this breaks the request to robohash (returns HTTP 500).
I suppose you could deploy an intermediate service that does the necessary URL translations and queries on your behalf (which caching of course), but that seems a little overkill.
That post is from 2016. The code behind Gravatar integration has change several times since then. It used to not be specific to Gravatar and you could specific arbitrary URL prefix and suffix. That is no longer the case.
Based on what I understand from current code, it is not currently possible out of the box. If you really really need robohash support, some of the options I can think of are: