[release/7.0-rc1] Use buffer.Length as the source of truth for ReadDirectoryChanges' nBufferLength #74023
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Backport of #73970 to release/7.0-rc1
/cc @jozkee
Customer Impact
Testing
Risk
IMPORTANT: If this change touches code that ships in a NuGet package, please make certain that you have added any necessary package authoring and gotten it explicitly reviewed.