Require individual MLS project keys and deprecate the fallback key

I manage the MLS team at Mozilla. Here's a graph that shows the number of geolocate requests to MLS for 2020 by all parties, including Geoclue:

Geoclue usage appears to be really taking off. Congrats!

Most people don't set their own MLS key when using Geoclue, relying instead on the fallback default. The "Other" bucket on that graph captures the whitelisted keys that may or may not be using Geoclue, but it's line noise in comparison to Geoclue's usage.

Mozilla cares about this for a few reasons:

1. We think MLS is a valuable, open service, and the service needs to be sustainable in order to continue to exist.
2. We like to understand where growth is coming from so we can invest resources where appropriate.
3. We want to make sure commercial entities are not abusing MLS via Geoclue and effectively getting a free ride.

My proposal is to deprecate the default key in Geoclue and give downstream users a good window (1 year?) in which to apply for their own MLS keys. Any Gnome or OSS apps that currently rely on Geoclue would effectively be rubber-stamped when they applied for their own MLS keys. At the end of the deprecation window, Mozilla would throttle the old key at a suitable threshold (probably 1M requests/day) so that old/unmaintained apps in the wild wouldn't be completely unsupported.

Happy to answer questions.