Infrastructure - Rollout (April 2020)

[ViktorHofer]

Overview

We'll use this issue to announce planned Infrastructure changes that will impact the developer workflow in dotnet/runtime.

Date	Change	Impact
April, 6th	#33553	libraries -buildtests action will be removed. Use -subset libtests instead. Minimum sdk version will be upgraded, install from https://github.com/dotnet/installer#installers-and-binaries.
April, 6th	#33756	Root helper build scripts will be deleted: coreclr.cmd/sh, libraries.cmd/sh, installer.cmd/sh and mono.cmd/sh. Use build.cmd/sh -subsetcategory instead.
April, 7th	#34663	-subsetcategory switch will be removed in favor of -subset (or unnamed argument), e.g. build.cmd/sh -subset clr+libs+libs.tests.