Bump Touch.Unit and MonoTouch.Dialog. (#19590)

New commits in migueldeicaza/MonoTouch.Dialog:

* migueldeicaza/MonoTouch.Dialog@5898074 [MonoTouch.Dialog] Use 'BundledNETCoreAppTargetFrameworkVersion' in TargetFramework to avoid having to bump every year.

Removed commits from migueldeicaza/MonoTouch.Dialog:

* migueldeicaza/MonoTouch.Dialog@f500f9a Bump to .NET 8.

Diff: https://github.com/migueldeicaza/MonoTouch.Dialog/compare/f500f9a48dbf856c693b255078e1507cf1e8edde..5898074d6e62109c85e587977230c1a521cb9bb2

New commits in spouliot/Touch.Unit:

* xamarin/Touch.Unit@92a0726 [Touch.Client] Use 'BundledNETCoreAppTargetFrameworkVersion' in TargetFramework to avoid having to bump every year.
* xamarin/Touch.Unit@afbf9d4 [Touch.Client] Silence numerous warnings about using native API too early, where it's not supported, or when obsoleted.
* xamarin/Touch.Unit@455abcd Bump to .NET 8.
* xamarin/Touch.Unit@2316d4a [NUnitLite] Fix a few warnings in .NET for TouchRunner.

Removed commits from spouliot/Touch.Unit:

* xamarin/Touch.Unit@5739ade Bump to .NET 8.

Diff: https://github.com/spouliot/Touch.Unit/compare/5739adefc261b7bfeb70ff718c7b2d97bdfa42ee..92a072683b69b2f61f235a7a249c2c7f261236d4

.gitmodules

@@ -9,15 +9,15 @@
 [submodule "external/Touch.Unit"]
     path = external/Touch.Unit
     url = ../../spouliot/Touch.Unit.git
-    branch = net8.0
+    branch = main
 [submodule "external/Xamarin.MacDev"]
     path = external/Xamarin.MacDev
     url = ../../xamarin/Xamarin.MacDev
     branch = main
 [submodule "external/MonoTouch.Dialog"]
     path = external/MonoTouch.Dialog
     url = ../../migueldeicaza/MonoTouch.Dialog
-    branch = net8.0
+    branch = dotnet
 [submodule "api-tools"]
     path = external/api-tools
     url = ../../rolfbjarne/api-tools