Update winui2.md to use SetEnvironmentVariable from Environment class for setting default background colour

[SnowNooDLe]

Rendered article section for review:
https://review.learn.microsoft.com/microsoft-edge/webview2/get-started/winui2?branch=pr-en-us-2863#setting-defaultbackgroundcolor
[GitHub's preview]
[Before]

fixes #2862

Updating the document to use the existing method, SetEnvironmentVariable instead of the SetVariable in the section of setting the default background colour.

Edit: It doesn't seem like I can link the issue I just opened. But this is related to #2862

By looking at the Environment class, it doesn't seem like it has the method named SetVariable, and I can see the usage of SetEnvironmentVariable with SmartScreen.

AB#46964293

[learn-build-service-prod]

Learn Build status updates of commit 58031f5:

✅ Validation status: passed

File	Status	Preview URL	Details
microsoft-edge/webview2/get-started/winui2.md	✅Succeeded	View

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[mikehoffms]

@SnowNooDLe - "Edit: It doesn't seem like I can link the issue I just opened. But this is related to [hash]2862" - the Issue says, at bottom, that you linked to it. I was able to link to the Issue two ways:

• Write "fixes [hash]1234" in topmost comment at PR's main page.
• PR's main page: Development section - select the Issue. d/k if you have that UI available.

[mikehoffms]

I requested that the PR Admin merge this PR.

[SnowNooDLe]

Thanks team :)