Add C# artifacts to `.gitignore` for `multi-language-repo` #4922

Job	Run time
Diagnostic export (macos-latest, linked)	1m 1s
Diagnostic export (macos-latest, nightly-latest)	1m 32s
Diagnostic export (windows-latest, linked)	3m 3s
Diagnostic export (ubuntu-latest, nightly-latest)	1m 6s
Diagnostic export (windows-latest, nightly-latest)	3m 34s
Diagnostic export (ubuntu-latest, linked)	58s
	11m 14s

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