Update build requirements in readme (#476)

* Update build requirements in readme

Updates the build requirements section of the repo README, bumping .NET 6 to 8 and adding the minimum Windows 10 Version the toolkit can be built on.

Tested against:
- Windows 10 Version 1809 Build 17763
  - With Windows SDK 17763 (doesn't build)
  - With Windows SDK 17763 and 19041 (doesn't build)
- Windows 10 Version 21H1 Build 19043
  - With Windows SDK 17763 (doesn't build)
  - With Windows SDK 17763 and 19041 (uwp and wasdk build)

* Update ReadMe.md

* Reorder build requirements to match Labs

* Add Migration Link to ReadMe

---------

Co-authored-by: Michael Hawker MSFT (XAML Llama) <24302614+michael-hawker@users.noreply.github.com>

Author: Arlodotexe

ReadMe.md

@@ -13,10 +13,12 @@ _Building something cool? Want to engage with other developers? Want to contribu
 
 ### [Try out our Sample Gallery from the Microsoft Store](https://aka.ms/windowstoolkitapp)
 
-Want to see the toolkit in action before jumping into the code? Download and play with the [Windows Community Toolkit Gallery](https://www.microsoft.com/store/apps/9nblggh4tlcq) from the Store.
+Want to see the toolkit in action before jumping into the code? Download and play with the [Windows Community Toolkit Gallery](https://aka.ms/windowstoolkitapp) from the Store.
 
 Please read the [Getting Started with the Windows Community Toolkit](https://docs.microsoft.com/dotnet/communitytoolkit/windows/getting-started) page for more detailed information about using the toolkit.
 
+If you're updating from a pre-8.x version of the Windows Community Toolkit, see [our migration notes here](https://aka.ms/toolkit/windows/migration).
+
 ### Windows Community Toolkit Labs
 
 Have an idea for a new feature? Want to checkout the latest things being built. _[Then head over to Windows Community Toolkit Labs](https://aka.ms/toolkit/labs/windows)_.
@@ -34,9 +36,11 @@ git clone --recurse-submodules https://github.com/CommunityToolkit/Windows.git
 ## Build Requirements
 
 - Visual Studio 2022 (UWP & Desktop Workloads for .NET)
-- .NET 6 SDK
-- Windows App SDK
-- Windows SDK 19041
+- .NET 8 SDK
+- Windows 10 SDK, version 2004 (10.0.19041.0)
+- Windows 10 21H1 (Build 19043) or greater
+- Run `dotnet tool restore` from the project root to install SlnGen
+- Run build scripts from the [Developer Command Prompt for Visual Studio](https://learn.microsoft.com/visualstudio/ide/reference/command-prompt-powershell) or from elsewhere after adding `MSBuild.exe` to your PATH
 
 ## 🚀 Contribution