[dotnet] Use net5.0-[ios|tvos|watchos|macos] TargetFrameworks. (#9532)

* [dotnet] Set TargetPlatformSupported when the right TargetPlatformIdentifier is used.

* [dotnet] Generate a list of valid OS versions for each platform, and add it to the SupportedTargetPlatform item group.

The generated files: https://gist.github.com/rolfbjarne/765c4e4b38b1b017380c9378d9666317

* [dotnet] Define and set the default platform version if it's not set in the TargetFramework variable.

* [dotnet] Switch to using the new-style TargetFramework values.

This includes bumping spouliot/Touch.Unit to get new-style TargetFramework values for Touch.Client.

* xamarin/Touch.Unit@89afaf7 [Touch.Client] Use the right TargetFrameworks for watchOS and macOS as well. (#92)
* xamarin/Touch.Unit@fd0e576 [Touch.Client] Use the right TargetFrameworks. (#91)
* xamarin/Touch.Unit@40f47db [Touch.Client] Add a macOS and watchOS version for .NET. (#90)
* xamarin/Touch.Unit@1d4b8c0 Add .gitignore for NuGet.config and global.json. (#89)
* xamarin/Touch.Unit@49441f3 Call `mlaunch` instead of `mtouch` (#88)
* xamarin/Touch.Unit@b157cf4 Fix a few markdown issues found by markdownlint. (#87)

Diff: https://github.com/spouliot/Touch.Unit/compare/d7f55a61673e18ae5a8f9628600081a54b4e074c..89afaf7e05a942c87231cf5601b3a5b59640e218

* [dotnet] Document the script that generates the lists of the target platform versions a little bit better.

* [dotnet] Make the [Platform]SupportedTargetPlatform variables public.

This matches how Android and Windows do it:

* dotnet/android#5007
* https://github.com/dotnet/sdk/blob/18ee4eac8b3abe6d554d2e0c39d8952da0f23ce5/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.WindowsSupportedTargetPlatforms.props

* [xharness] Update the TargetFramework value when creating project variations.

Author: rolfbjarne

dotnet/.gitignore

@@ -1,2 +1,3 @@
 nupkgs
+Microsoft.*.Sdk/targets/Microsoft.*.Sdk.SupportedTargetPlatforms.props
 

dotnet/Makefile

@@ -7,6 +7,7 @@ define DefineTargets
 $(1)_NUGET_TARGETS = \
 	$(DOTNET_DESTDIR)/Microsoft.$(1).Sdk/Sdk/Sdk.targets \
 	$(DOTNET_DESTDIR)/Microsoft.$(1).Sdk/Sdk/Sdk.props \
+	$(DOTNET_DESTDIR)/Microsoft.$(1).Sdk/targets/Microsoft.$(1).Sdk.SupportedTargetPlatforms.props \
 	$(DOTNET_DESTDIR)/Microsoft.$(1).Sdk/targets/Microsoft.$(1).Sdk.DefaultItems.props \
 	$(DOTNET_DESTDIR)/Microsoft.$(1).Sdk/targets/Microsoft.$(1).Sdk.props \
 	$(DOTNET_DESTDIR)/Microsoft.$(1).Sdk/targets/Microsoft.$(1).Sdk.targets \
@@ -61,8 +62,17 @@ targets/%.props: targets/%.template.props Makefile $(TOP)/Make.config.inc $(TOP)
 		-e "s/@CURRENT_HASH_LONG@/$(CURRENT_HASH_LONG)/g" \
 	$< > $@
 
+define SupportedTargetPlatforms
+Microsoft.$(1).Sdk/targets/Microsoft.$(1).Sdk.SupportedTargetPlatforms.props: $(TOP)/Versions-ios.plist.in $(TOP)/Versions-mac.plist.in Makefile ./generate-target-platforms.csharp Makefile
+	$(Q) rm -f $$@.tmp
+	$(Q) ./generate-target-platforms.csharp $(1) $$@.tmp
+	$(Q) mv $$@.tmp $$@
+endef
+$(foreach platform,$(DOTNET_PLATFORMS),$(eval $(call SupportedTargetPlatforms,$(platform))))
+
 TEMPLATED_FILES = \
 	targets/Xamarin.Shared.Sdk.Versions.props \
+	$(foreach platform,$(DOTNET_PLATFORMS),Microsoft.$(platform).Sdk/targets/Microsoft.$(platform).Sdk.SupportedTargetPlatforms.props) \
 
 nupkgs/$(IOS_NUGET).%.nupkg: CURRENT_VERSION_NO_METADATA=$(IOS_NUGET_VERSION_NO_METADATA)
 nupkgs/$(TVOS_NUGET).%.nupkg: CURRENT_VERSION_NO_METADATA=$(TVOS_NUGET_VERSION_NO_METADATA)