Add C# artifacts to `.gitignore` for `multi-language-repo` #7637

Job	Run time
Packaging: Action input (ubuntu-latest, default)	1m 52s
Packaging: Action input (macos-latest, linked)	2m 45s
Packaging: Action input (windows-latest, nightly-latest)	4m 20s
Packaging: Action input (ubuntu-latest, linked)	1m 53s
Packaging: Action input (windows-latest, linked)	4m 13s
Packaging: Action input (ubuntu-latest, nightly-latest)	2m 0s
Packaging: Action input (macos-latest, nightly-latest)	2m 29s
Packaging: Action input (windows-latest, default)	5m 18s
Packaging: Action input (macos-latest, default)	1m 35s
	26m 25s

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