Add C# artifacts to `.gitignore` for `multi-language-repo` #4500

Job	Run time
Resolve environment (macos-latest, default)	1m 36s
Resolve environment (ubuntu-latest, default)	33s
Resolve environment (ubuntu-latest, linked)	36s
Resolve environment (macos-latest, nightly-latest)	2m 10s
Resolve environment (ubuntu-latest, nightly-latest)	34s
Resolve environment (macos-latest, linked)	2m 6s
Resolve environment (windows-latest, linked)	2m 43s
Resolve environment (windows-latest, default)	2m 15s
Resolve environment (windows-latest, nightly-latest)	4m 21s
	16m 54s

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