Meta: move CI from Buildbot to Taskcluster

[SimonSapin]

Previously

servo/saltfs#559 (comment)

Done

Docs: etc/taskcluster/README.md

• #21776
  • In-tree decision task
  • In-tree Dockerfiles, with corresponding images built as needed and cached
  • CI task: tidy + Linux dev build + unit tests
  • CI tasks (implemented but disabled for now): Linux release build + WPT
  • Daily task: Linux dev with Rust Nigthly
• servo/saltfs#888
  • Make Homu also gate PRs on Taskcluster
• #21835
  • Android ARMv7 build task
• #21863
  • Configure Windows 2016 workers
  • Windows build task
• #22046
  • Android x86 build task
  • Configure Packet.net worker
  • Android x86 emulator tests task
• #22185
  • Configure Macstadium worker
  • macOS dev build + unit tests task
• #22233 Treeherder integration: https://treeherder.mozilla.org/#/jobs?repo=servo-auto
• mozilla/treeherder#4320 Use the GitHub events API to find the date of a push
• #22381 + #22436
  • Build docs and upload to doc.servo.org on Taskcluster
  • Support for trychoosers, part 1
• #22419 Move "dev" builds and unit tests from Buildbot to Taskcluster
• #22424 Documentation on Treeherder
• servo/saltfs#925
  • Trychoosers part 2
  • Disable some Buildbot builders
• mozilla/treeherder#4371 Show all try-* branches on https://treeherder.mozilla.org/#/jobs?repo=servo-try
• #22459 Move macOS WPT chunk 1 (only) from Buildbot to Taskcluster
• #22464 Build Servo Nigthly for Linux, upload to S3
• #22435 Move Linux WPT from Buildbot to Taskcluster

To do

Tasks currently only on Buildbot:

• CI
  • Android build from macOS
  • Magic Leap build
  • Linux ARM32 & ARM64 builds
• Daily
  • Other Nightly builds uploads: macOS, Windows, Android, Magic Leap
  • WPT upstream sync

When we’re happy with the stability of a given task on Taskcluster:

• Remove the corresponding job/builder from Buildbot configuration
• If any, decommission workers that are now unused
• Remove unused entry in buildbot_steps.yml