EES-5084 Add ProblemDetailsResultFilter to admin

This fixes incorrectly formatted validation error responses.

src/GovUk.Education.ExploreEducationStatistics.Admin/Startup.cs

@@ -146,6 +146,7 @@ public virtual void ConfigureServices(IServiceCollection services)
                 {
                     options.Filters.Add(new AuthorizeFilter(SecurityPolicies.RegisteredUser.ToString()));
                     options.Filters.Add(new OperationCancelledExceptionFilter());
+                    options.Filters.Add(new ProblemDetailsResultFilter());
                     options.EnableEndpointRouting = false;
                     options.AllowEmptyInputInBodyModelBinding = true;
                 })