[csharp] fix system.web warning and removed unneeded fqn

[filipe-silva]

Refactor validatable.mustache moved fqn types to using directive for cleaner code Fix netcore_project.mustache conditional "System.Web", more details: https://learn.microsoft.com/en-us/aspnet/core/migration/inc/adapters?view=aspnetcore-7.0

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@mandrean @shibayan @Blackclaws @lucamazzanti @iBicha

[wing328]

circlci node 2 failure not related to this change.

[wing328]

thanks for the PR

cc @devhl-labs

[wing328]

if users have a model/schema called ValidationResult somehow which should result in compilation errors after this change, they can use the model mapping feature to workaround the issue: https://github.com/openapitools/openapi-generator/blob/master/docs/customization.md#name-mapping