Bump up osx machines to 10.13
Bump up osx machines to 10.13

Based on https://github.com/dotnet/coreclr/issues/10337

rainersigwald/msbuild,chipitsine/msbuild,AndyGerlicher/msbuild,jeffkl/msbuild,mono/msbuild,AndyGerlicher/msbuild,jeffkl/msbuild,mono/msbuild,mono/msbuild,cdmihai/msbuild,chipitsine/msbuild,AndyGerlicher/msbuild,rainersigwald/msbuild,Microsoft/msbuild,cdmihai/msbuild,rainersigwald/msbuild,Microsoft/msbuild,sean-gilliam/msbuild,jeffkl/msbuild,sean-gilliam/msbuild,Microsoft/msbuild,chipitsine/msbuild,chipitsine/msbuild,sean-gilliam/msbuild,rainersigwald/msbuild,cdmihai/msbuild,cdmihai/msbuild,sean-gilliam/msbuild,mono/msbuild,Microsoft/msbuild,AndyGerlicher/msbuild,jeffkl/msbuild