Add missing using statement for ExcludeFromCodeCoverage attribute

Add using System.Diagnostics.CodeAnalysis to migration files that use [ExcludeFromCodeCoverage] attribute.

Co-authored-by: kamilbaczek <74410956+kamilbaczek@users.noreply.github.com>

Author: Copilot

Chapter-3-microservice-extraction/Fitnet.Contracts/Src/Fitnet.Contracts.Infrastructure/Database/Migrations/20230601184153_MakeSignedAtColumnNullable.cs

@@ -3,6 +3,7 @@
 namespace EvolutionaryArchitecture.Fitnet.Contracts.Data.Database.Migrations;
 
 using System;
+using System.Diagnostics.CodeAnalysis;
 using Microsoft.EntityFrameworkCore.Migrations;
 
 [ExcludeFromCodeCoverage]

Chapter-3-microservice-extraction/Fitnet.Contracts/Src/Fitnet.Contracts.Infrastructure/Database/Migrations/20230601184328_AddCustomerIdColumn.cs

@@ -3,6 +3,7 @@
 namespace EvolutionaryArchitecture.Fitnet.Contracts.Data.Database.Migrations;
 
 using System;
+using System.Diagnostics.CodeAnalysis;
 using Microsoft.EntityFrameworkCore.Migrations;
 
 [ExcludeFromCodeCoverage]

Chapter-3-microservice-extraction/Fitnet.Contracts/Src/Fitnet.Contracts.Infrastructure/Database/Migrations/20230624171216_ContractsAddColumnsToSupportContractExpiration.cs

@@ -3,6 +3,7 @@
 namespace EvolutionaryArchitecture.Fitnet.Contracts.Data.Database.Migrations;
 
 using System;
+using System.Diagnostics.CodeAnalysis;
 using Microsoft.EntityFrameworkCore.Migrations;
 
 [ExcludeFromCodeCoverage]