Update CommunityToolkit.Maui.Markup Docs for .NET 9

[TheCodeTraveler]

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DO NOT MERGE THIS UNTIL THIS PR ON COMMUNITYTOOLKIT.MAUI.MARKUP HAS BEEN MERGED: CommunityToolkit/Maui.Markup#324

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This PR updates the CommunityToolkit.Maui.Markup docs for .NET 9.

In .NET 9, using a string path for bindings is no longer supported. We've also removed all [Obsoelete] extension methods (e.g. ClickGestureRecognizer).

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Learn Build status updates of commit 81e8415:

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Learn Build status updates of commit 5d2cabe:

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Learn Build status updates of commit fcbfe18:

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