Can't update core with composer: "Your requirements could not be resolved to an installable set of packages"
The solution in my case was to delete the vendor directory and delete composer.lock.

I am not totally sure I needed to delete the vendor directory, because I tried that first without deleting composer.lock, and it didn't work.  Deleting composer.lock was necessary.

I also changed the minimum-stability to "stable" before deleting vendor and composer.lock, but I doubt that was necessary.  In any case, I now have...


  "minimum-stability": "stable",