[release/6.0] Use Ubuntu 18.04 and dotnet-buildtools-prereq docker images for enterprise linux pipeline#63014

safern merged 5 commits intorelease/6.0dotnet/runtime:release/6.0from

backport/pr-61001-to-release/6.0dotnet/runtime:backport/pr-61001-to-release/6.0Copy head branch name to clipboard