Refactor ServiceProvider with conditional compilation

Updated the `ServiceProvider` field in the `ReC.Client` namespace to use conditional compilation for framework-specific behavior:
- Added `NET8_0_OR_GREATER` directive to define `ServiceProvider` as nullable (`IServiceProvider?`) for .NET 8.0 or greater.
- Retained non-nullable `IServiceProvider` for other frameworks.
- Removed redundant `#if nullable` directives to simplify the code.
- Streamlined initialization logic for better clarity and maintainability.

src/ReC.Client/ReCClient.cs

@@ -48,15 +48,11 @@ namespace ReC.Client
         #region Static
         private static readonly IServiceCollection Services = new ServiceCollection();
 
-        private static IServiceProvider
-#if nullable
-            ?
+#if NET8_0_OR_GREATER
+        private static IServiceProvider? ServiceProvider = null;
+#else
+        private static IServiceProvider ServiceProvider = null;
 #endif
-            ServiceProvider
-#if nullable
-            = null
-#endif
-            ;
 
         public static void BuildStaticClient(string apiUri)
         {