Remove obsolete API

In the next release, we might finally drop the support for the following obsolete methods

```c#
  [Obsolete]
  Task<T?> GetBySpecAsync(ISpecification<T> specification, CancellationToken cancellationToken = default);
  [Obsolete]
  Task<TResult?> GetBySpecAsync<TResult>(ISpecification<T, TResult> specification, CancellationToken cancellationToken = default);
```