Removing older images in Azure Pipelines hosted pools

This post announces the removal of specific older images from Azure Pipelines hosted pools (Windows Server 2012R2 with VS 2015, macOS High Sierra 10.13, Windows Server Core 1803) due to low usage and to make room for new updates. It provides migration guidance for users of these images, including switching to newer hosted images (e.g., `vs2017-win2016`, `windows-2019`, `macOS-10.14`, `macOS-10.15`), updating Xcode versions, or using self-hosted agents. It also details a brownout schedule to alert users of affected builds.