Kill processes after completing build and before publishing test results or artifacts (#7228)

- aspnet/AspNetCore-Internal#1735
- port aspnet/Extensions@e2bb8ac1a0 fix into this repo
  - change `default-build.yml` instead of `azure-pipelines.yml`
  - make a couple of changes to `KillProcesses.ps1` (which I'll take back to Extensions)
- remove `ci-official.yml`