Set IDE0031 to suggestion in eng/CodeAnalysis.src.globalconfig (#119755)

Author: akoeplinger

eng/CodeAnalysis.src.globalconfig

@@ -1667,7 +1667,7 @@ dotnet_diagnostic.IDE0029.severity = warning
 dotnet_diagnostic.IDE0030.severity = warning
 
 # IDE0031: Use null propagation
-dotnet_diagnostic.IDE0031.severity = warning
+dotnet_diagnostic.IDE0031.severity = suggestion
 
 # IDE0032: Use auto property
 dotnet_diagnostic.IDE0032.severity = silent