Update dependency Avalonia.Diagnostics to v0.10.21

This MR contains the following updates:

Package	Type	Update	Change
Avalonia.Diagnostics (source)	nuget	patch	0.10.20 -> 0.10.21

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Release Notes

AvaloniaUI/Avalonia

v0.10.21

Compare Source

• Remove msbuild debug code, causing a "test.bin" file to be left next to the main project.

https://www.nuget.org/packages/Avalonia/0.10.21

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