infra: Remove remaining references to 'master'. These are all now replaced with 'main'. Bug: angleproject:6685 Change-Id: I36cb17625c44d06689f7debf004abd3dd016265b Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3276517 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>