Update spec URLs for Class Fields (#10923)
Update spec URLs for Class Fields (#10923)

The Class Fields proposal has been merged into the ES spec itself. So
this change updates the spec URL for the class-fields features to their
locations in the ES spec.
mdn/browser-compat-data,mdn/browser-compat-data,chrisdavidmills/browser-compat-data,chrisdavidmills/browser-compat-data