[release/6.0] Update Docker images, queues, etc. (#38563)

- cherry-pick of 64c37113
- also realigns a couple of files w/ 'main' (because that was easier)
  - e.g. 30d095bb and most of 34e0b982

- part of dotnet/aspnetcore-internal#3950
  - also touches on #36032
- update Helix queues from Alpine 3.12 to 3.14, OSX 10.14 to 10.15, and (for Arm64) Debian 9 to 11
  - use OSX 11.00 when testing PRs and rolling builds; reduce 10.15 usage to scheduled runs
  - remove overlap (all 3 queues) between PRs / rolling builds and scheduled runs
  - remove quarantined-pr / quarantined-tests overlap
- build source-index on `windows-latest` (not `vs2017-win2016`)
- update build and Helix Docker images to latest tags

- skip a test class on macOS 10.15
  - #38542

nits:
- don't skip unused Helix queues
- remove versions from pipeline job display names
  - some were already outdated; rest will be confusing in the future
- remove most comments about unused Helix queues