Review Bot 2.0 introduced a setting for defining the Review Board

servers that Review Bot may perform API requests on. This setting was

named review_board_servers, which isn't very consistent with how we

typically identify Review Board in key/filename form. A more consistent

name would be reviewboard_servers.
This change updates our code to use the new setting, and to migrate the

old setting to the new one, with a deprecation warning logged.
Documentation has been updated to reference this change, and to document

it in the Upgrading page.

Manually tested with the old and new setting. Saw a deprecation notice

for the old one, and saw that the setting transitioned over.
Unit tests pass.
Viewed the documentation and checked for bad links, spelling errors, and

build errors.