Azure Pipelines hosted pools updates

This post announces updates to Azure Pipelines hosted pools, including the upcoming update of `ubuntu-latest` to `ubuntu-18.04` (from `ubuntu-16.04`), the release of a new `macOS-10.15` (Catalina) image, and the update of `macOS-latest` to point to `macOS-10.15` (from `macOS-10.14`). It also details the removal of `vs2015-win2012r2`, `macOS-10.13`, and `win1803` images from hosted pools in March 2020 to facilitate the rollout of newer image versions.