* Clean out Ubuntu 16.04 testing
- use Ubuntu 18.04 agents for testing in Docker containers on Helix

nits:
- clean up a few old mentions of Centos and Fedora testing
- fix `%(Platform)` metadata for OSX `@(HelixAvailableTargetQueue)` items
- don't mention unused internal queues in `$(TestDependsOnPlaywright)` update of `$(SkipHelixQueues)`
- sort remaining queues alphabetically in `$(SkipHelixQueues)`