Update WelcomeToMRTK.md

The Unity development journey on docs.microsoft.com currently supports MRTK 2.4 (this will change soon).

Author: polar-kev

Documentation/WelcomeToMRTK.md

@@ -19,7 +19,7 @@ The Mixed Reality Toolkit (MRTK) is a cross-platform toolkit for building Mixed
 If you're new to MRTK or Mixed Reality development in Unity, we recommend you start at the beginning of our [Unity development journey](https://docs.microsoft.com/windows/mixed-reality/unity-development-overview?tabs=mrtk%2Chl2). The Unity development journey is the **recommended starting point for MRTK**, specifically created to walk you through the installation, core concepts, and usage of MRTK in Unity. 
 
 > [!CAUTION]
-> The Unity development journey currently uses **MRTK version 2.5.0** and **Unity 2019.4**. If you're working with other configurations it's still recommended that you start there, but you can also refer to the [installation instructions](Installation.md).
+> The Unity development journey currently uses **MRTK version 2.4.0** and **Unity 2019.4**. If you're working with other configurations it's still recommended that you start there, but you can also refer to the [installation instructions](Installation.md).
 
 ## Documentation versioning
 
@@ -32,4 +32,4 @@ We have complete documentation for all release versions. The **mrtk_development*
 If you run into issues caused by MRTK or otherwise have questions about how to do something, there are a few resources that can help:
 
 * For bug reports, please [file an issue](https://github.com/microsoft/MixedRealityToolkit-Unity/issues/new/choose) on the GitHub repo.
-* For questions, please reach out on either [StackOverflow](https://stackoverflow.com/questions/tagged/mrtk) or the [mixed-reality-toolkit channel](https://holodevelopers.slack.com/messages/C2H4HT858) on Slack. You can join the Slack community via the [automatic invitation sender](https://holodevelopersslack.azurewebsites.net/).
+* For questions, please reach out on either [StackOverflow](https://stackoverflow.com/questions/tagged/mrtk) or the [mixed-reality-toolkit channel](https://holodevelopers.slack.com/messages/C2H4HT858) on Slack. You can join the Slack community via the [automatic invitation sender](https://holodevelopersslack.azurewebsites.net/).