Updates to the `Build and Release Tools` and Mozharness modules

[Aki Sasaki]

Hi all,

The Core: Build and Release Tools and Mozharness modules need new owners.

For Core: Build and Release Tools, I propose moving myself to owner emeritus, adding Julien Cristau as owner, and Heitor Neiva and Ben Hearsum as peers. Julien is an expert in updates; Heitor spent significant time improving the update-verify code last year; Ben was the owner previous to me, and he has agreed to be a peer.

For Mozharness, I propose moving myself to owner emeritus, Tom Prince and Justin Wood to peers emeritus, and adding Geoff Brown as owner and Joel Maher as peer. Geoff and Joel both know mozharness and are active in creating and reviewing patches.

Barring any objections, I'd like to land the following revision to update the module next week:
https://phabricator.services.mozilla.com/D164479

Please let me know if you have any suggestions or concerns!
Aki