Revert WinAppSdkTargetFramework version from 22621 to 19041.

[Arlodotexe]

Prerequisite PR CommunityToolkit/Windows#540

Since Win2d 1.1.1 was the reason we originally bumped our Windows App SDK TFM to 22621 (see #161, microsoft/Win2D#943) and 1.3.0 has reverted to 19041, we can reduce our TFM back down to 19041.

[michael-hawker]

Think there's a couple of more places we need to modify, no? https://github.com/search?q=repo%3ACommunityToolkit%2FTooling-Windows-Submodule%2022621&type=code

[michael-hawker]

We should still have something like Morten suggested for if folks try and use a lower TFM than 19041 too, eh? (And then we can decide if we hot-fix 8.1 to use that for 22621, eh?)

[Arlodotexe]

Think there's a couple of more places we need to modify, no? https://github.com/search?q=repo%3ACommunityToolkit%2FTooling-Windows-Submodule%2022621&type=code

No, at least not for this PR. There are four instances of use in the search results link provided:

• MultiTarget/EnabledTargetFrameworks.props, handled by this PR.
• MultiTarget/AvailableTargetFrameworks.props, handled by this PR.
• In the WindowsSdkPackageVersion found in MultiTarget/WinUI.Extra.props.
• In the version for Microsoft.Windows.SDK.BuildTools in MultiTarget/PackageReferences/WinAppSdk.props.

The last two are slated for removal entirely as per @Sergio0694's advice in #217 (comment) and #217 (comment), which will be done in this PR.

[Arlodotexe]

We should still have something like Morten suggested for if folks try and use a lower TFM than 19041 too, eh? (And then we can decide if we hot-fix 8.1 to use that for 22621, eh?)

Yes, though I'm still working through many of the nuances to make sure we can apply it to all toolkit components automatically, instead of manually doing it one-by-one. Can discuss in more detail in a dedicated ticket, or over on Discord in the CommunityToolkit channel.

Here, we're just reducing the TFMs back down, whether we decide to hotfix or not.