[src] Improve the generated project files a little bit. (#17812)

* Use the right TFM.
* Compute the right assembly name.

src/Makefile

@@ -1352,6 +1352,7 @@ define DotNetProjectFiles
 $(DOTNET_BUILD_DIR)/projects/$(1)/$(1).csproj: dotnet.tmpl.csproj Makefile $$(wildcard $(CURDIR)/*.sources)
 	@mkdir -p $$(dir $$@)
 	@sed \
+		-e 's*%DOTNET_TFM%*$(DOTNET_TFM)*' \
 		-e 's*%PLATFORM%*$(1)*' \
 		-e 's*<!--%FILES%-->*$$(foreach file,$$($(2)_DOTNET_SOURCES),<Compile Include="../../../../$$(file)" Link="sources/$$(file)" />)*' \
 		-e 's*<!--%APIS%-->*$$(foreach file,$$($(2)_DOTNET_APIS),<None Include="../../../../$$(file)" Link="apis/$$(file)" />)*' \

src/dotnet.tmpl.csproj

@@ -3,10 +3,10 @@
   <!-- This is a project file to load in the IDE to get code completion, etc. It's not used for building the product assemblies, that happens in the makefile -->
   <PropertyGroup>
     <_TargetPlatform>%PLATFORM%</_TargetPlatform>
-    <TargetFramework>net6.0</TargetFramework>
+    <TargetFramework>%DOTNET_TFM%</TargetFramework>
     <TargetFrameworkIdentifier>.NETCoreApp</TargetFrameworkIdentifier> <!-- this line shouldn't be necessary, but VSMac on one of my machines won't load the project otherwise -->
     <OutputType>Library</OutputType>
-    <AssemblyName>Xamarin.iOS</AssemblyName>
+    <AssemblyName>Microsoft.%PLATFORM%</AssemblyName>
     <SignAssembly>true</SignAssembly>
     <AssemblyOriginatorKeyFile>..\..\..\..\..\product.snk</AssemblyOriginatorKeyFile>
     <LangVersion>latest</LangVersion>