chore(internal): suppress diagnostic for .netstandard2.0

Author: stainless-app[bot]

.editorconfig

@@ -29,3 +29,4 @@ dotnet_diagnostic.IDE0290.severity = none # Don't prefer primary constructors
 dotnet_diagnostic.IDE0057.severity = none # Caused by use of `.Substring(...)`
 dotnet_diagnostic.CA1866.severity = none # Caused by use of `.StartsWith(...)` with single character string
 dotnet_diagnostic.CA1847.severity = none # Caused by use of `.Contains(...)` with single character string
+dotnet_diagnostic.CA2263.severity = none # Caused by use of non-generic `Enum.IsDefined(...)`