[PSA] Changing the //third_party approval forum.

[Mike West]

Hello friendly chromium-dev@,

Previously, y'all sent requests to approve new //third_party dependencies to the ATLs. Now, please send them to a dedicated list: chromium-third-party-libraries@. That should clean up the ATLs' discussion list, and will make it easier for folks following third-party library additions to track those conversations. If you're one of those folks, please join!

We've also updated our documentation to hopefully make each approval conversation clearer and more efficient by highlighting questions that will be helpful to answer right away, as well as some useful trybot data provisional CLs could reveal.

Thanks, and happy third-partying!

-mike