Add change note

csharp/ql/lib/change-notes/2023-05-30-source-generators.md

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+category: majorAnalysis
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+* The extractor has been changed to run after the traced compiler call. This allows inspecting compiler generated files, such as the output of source generators. With this change, `.cshtml` files and their generated `.cshtml.g.cs` counterparts are extracted on dotnet 6 and above.