Add C# artifacts to `.gitignore` for `multi-language-repo` #10219

Job	Run time
analyze-actions	56s
check-codeql-versions	33s
analyze-javascript (ubuntu-24.04)	4m 2s
analyze-javascript (ubuntu-22.04)	4m 4s
analyze-javascript (windows-2022, linked)	6m 16s
analyze-javascript (macos-15)	4m 46s
analyze-javascript (windows-2022)	6m 12s
analyze-javascript (ubuntu-24.04, linked)	3m 59s
analyze-javascript (macos-13)	8m 26s
analyze-javascript (windows-2025)	6m 36s
analyze-javascript (macos-14, linked)	4m 22s
analyze-javascript (ubuntu-22.04, linked)	4m 19s
analyze-javascript (macos-14)	4m 43s
analyze-javascript (macos-13, linked)	7m 36s
analyze-javascript (macos-15, linked)	4m 8s
analyze-javascript (windows-2025, linked)	6m 16s
	1h 17m 14s

Provide feedback