HttpClient is not created and managed with IHttpClientFactory

[MS article](https://learn.microsoft.com/en-us/dotnet/architecture/microservices/implement-resilient-applications/use-httpclientfactory-to-implement-resilient-http-requests) on why you should not be managing your `HttpClient` manually and instead use `IHttpClientFactory`. See [here](https://github.com/Flagsmith/flagsmith-dotnet-client/blob/main/Flagsmith.FlagsmithClient/FlagsmithConfiguration.cs#L89) for line that should be updated.

Important tidbits here:
"_Another issue that developers run into is when using a shared instance of `HttpClient` in long-running processes. In a situation where the `HttpClient` is instantiated as a singleton or a static object, it fails to handle the DNS changes as described in this [issue](https://github.com/dotnet/runtime/issues/18348) of the dotnet/runtime GitHub repository._"

_"Though this class implements `IDisposable`, declaring and instantiating it within a using statement is not preferred because when the `HttpClient` object gets disposed of, the underlying socket is not immediately released, which can lead to a socket exhaustion problem. For more information about this issue, see the blog post [You're using `HttpClient` wrong and it's destabilizing your software](https://aspnetmonsters.com/2016/08/2016-08-27-httpclientwrong/)."_

Start with Flagsmith.FlagsmithClient/FlagsmithConfiguration.cs at line 89 and read the surrounding client construction and its callers. Compare that setup with the linked Microsoft IHttpClientFactory guidance; done means the client is created and managed through the factory rather than manually.

Written by the indexing model from the issue text.