Fix warnings #4015

Job	Run time
windows--dotnet6-and-legacyFramework	7m 5s
macOS--dotnet6-and-mono	17m 5s
linux-github--dotnet-and-newmono	8m 24s
linux-github--dotnet-and-mono	4m 56s
macOS--dotnet6-only	7m 12s
macOS--mono-only	3m 50s
windows--dotnet6-only	6m 24s
windows--legacyFramework-only	12m 24s
linux-vanilla--stockdotnet6-and-newmono	6m 12s
linux-vanilla--newmono-only	3m 14s
linux-vanilla--stockdotnet6-and-stockmono	7m 2s
linux-vanilla--stockdotnet6-only	5m 17s
linux-vanilla--stockmono-only	1m 44s
conventions	0s
snap_pkg	0s
snap_pkg_beta	0s
	1h 30m 49s

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