Bump $(ProductVersion) to 11.3.99 (#5876)

(Commercial) Xamarin.Android v11.3 is being tracked in the
[xamarin-android/d16-10][0] branch, which was branched from
commit eac74eb.

Bump `$(ProductVersion)` to 11.3.99 to track development progress of
the *next* version of Xamarin.Android.

Additionally, bump various submodules to their "main" equivalents
to mirror the d16-10 branch commit 496a43a, and update `.gitmodules`
so that the correct branch name is used:

Changes: mono/debugger-libs@8bf09ce...08268b7

  * mono/debugger-libs@08268b7: Add Module.ApplyChanges (#342)
  * mono/debugger-libs@1b5a55a: Fix assembly version after converting projects to SDK style

Changes: mono/opentk@933939d...50128f7

  * mono/opentk@50128f7a: Create Xamarin.Legacy.OpenTK Nuget Package (#28)
  * mono/opentk@72f4b085: Merge pull request #26 from dellis1972/ci
  * mono/opentk@b8fd3d2f: Fix Path issue
  * mono/opentk@62fd3b17: Clean up
  * mono/opentk@3a1a53c7: Add missing files. Update .gitignore
  * mono/opentk@e657cf71: Build with dotnet
  * mono/opentk@6e9a6160: Switch to using Xamarin.Legacy.Sdk
  * mono/opentk@a46a0668: rework
  * mono/opentk@cdae43e0: Initial Commit for Building Android

Changes: dotnet/android-tools@d92fc3e...c5732a0

  * dotnet/android-tools@c5732a0: [Xamarin.Android.Tools.AndroidSdk] Support Microsoft Dist JDK (#117)
  * dotnet/android-tools@52ef989: [Xamarin.Android.Tools.AndroidSdk] Fix CS0168 warning (#116)

[0]: https://github.com/xamarin/xamarin-android/commits/d16-10

.gitmodules

@@ -9,11 +9,11 @@
 [submodule "external/debugger-libs"]
     path = external/debugger-libs
     url = https://github.com/mono/debugger-libs
-    branch = master
+    branch = main
 [submodule "external/Java.Interop"]
     path = external/Java.Interop
     url = https://github.com/xamarin/java.interop.git
-    branch = master
+    branch = main
 [submodule "external/lz4"]
     path = external/lz4
     url = https://github.com/lz4/lz4.git
@@ -29,7 +29,7 @@
 [submodule "external/opentk"]
     path = external/opentk
     url = https://github.com/mono/opentk.git
-    branch = master
+    branch = main
 [submodule "external/proguard"]
     path = external/proguard
     url = https://github.com/Guardsquare/proguard.git

Directory.Build.props

@@ -13,7 +13,7 @@
   </PropertyGroup>
 
   <PropertyGroup>
-    <ProductVersion>11.2.99</ProductVersion>
+    <ProductVersion>11.3.99</ProductVersion>
     <!-- NuGet package version numbers. See Documentation/guides/OneDotNet.md.
          Rules:
          * Reset patch version (third number) to 100 every time either major or minor version is bumped.