Updated policy for adjust CI test platforms and related manifest conditions

[Joel Maher]

As I have been doing most of the work for CI upgrades, I would like to take the opportunity to share a more formalized policy for how we edit manifests when we are upgrading or adding new platforms for Firefox/Geckoview CI (mozilla-central, etc.).

In short, the current method involves a lot of try pushes, manifest/test editing, and repeat until everything is passing.  Once we create the changes required, it involves a lengthy review period (with no nagging it is often >2 weeks).  As outlined in the policy there will be a required review from #intermittent-reviewers, and all related review groups will be cc'd (not blocking reviews).  Test failures will fall into 2 categories:

 * pre-existing - will land as soon as r+

 * new failures - will land in 3 business days (allow test owners to weigh in)

Lastly, it will be customary to send an announcement out to dev.platform about an upcoming upgrade 2+ weeks prior to making the switch with an idea of what to expect, a meta bug, etc. 

This will allow us to get tests running faster on more relevant OS versions and platforms while spending our engineering time making our CI run faster, and provide more useful data.

If you have questions or concerns, please reach out on #firefox-ci in Matrix.

Thanks,

Joel (:jmaher)