upgrade mailgun driver & deprecate

[mtrezza]

Upgrades from the unmaintained mailgun-js to the official Mailgun mailgun.js driver.
The official driver works differently, so this is breaking change -> 2.0.0.

The reason for this bump is that the deprecated driver has a high level security issue and since this mail adapter is part of the parse-server repo, that repo also reports a high level vulnerability.

I did this PR just quickly, not sure whether this works, someone would have to try it out.

@davimacedo @dplewis Do we want to deprecate this repo? There are many mail adapters for Parse Server out there. This packages does not have any tests or even a changelog (just added one now in case we want to keep the repo). Not sure whether this is used in Parse Server tests anywhere though.

[davimacedo]

Maybe we should not only deprecate this one but also remove it from the parse-server dependencies.

[davimacedo]

I've just seen parse-community/parse-server#7320 :)

[mtrezza]

It is the only native Parse Server Mail Adapter, that's the only thing we'd have to consider. I'm fine with transferring the API Mail Adapter to the org at some point in the future (I believe its currently the most advanced mail adapter out there), but I keep it for now to experiment a bit with release automation.

[mtrezza]

I have removed all references to this mail adapter from Parse Server, in README and code.

I have moved the API Mail Adapter to the top of the list of suggested community mail adapters, in the hope that adoption increases and it matures further. To my knowledge it is also the only adapter that currently supports localization of Parse Server password reset and email verification emails in interplay with their localized pages.

@davimacedo How do we deprecate this? Just add a note to the README, and probably set it to "archived" in GitHub? Anything in npm?

[mtrezza]

Added a deprecation note to the README.