(Using XAMPP version 7.0.9 on Windows 10.) After 50 minutes of searching and trying things, it was a single line fix:
C:\xampp\php\php.ini and add the line:
curl .cainfo = "C:/xampp/apache/bin/curl-ca-bundle.crt"
Oddly I didn't even have to update curl-ca-bundle.crt to a new version. Just needed that setting set.