Improve nullability for TypeConverter.CanConvertTo (#63874)

Improve nullability for TypeConverter.CanConvertTo (#63874)

The nullability of parameter 'destinationType' in
TypeConverter.CanConvertTo(...) was changed from
not-nullable to nullable during the development of .NET 6.

Since a destination type supported by this TypeConverter
can never be null, a NotNullWhenAttribute is added to the
'destinationType' parameter when the result value of
TypeConverter.CanConvertTo(...) is 'true'.

Fix #63186