refactor: 💡 refactor

- **Upgraded to .NET 8**
- **Removed dependencies on RestSharp and Newtonsoft**
- **Replaced Newtonsoft with System.Text.Json for serialization**
- **Addressed issue of incompatible use of System.Text.Json and
    Newtonsoft**
- **Modernized TaxjarAPI with the following objectives:**
  - Made implementation injectable and testable
  - Utilized HttpClientFactory for best practices
  - Minimized reflection usage

- Created public `ITaxjarApi` interface for custom implementations
  and mocks
- Changed date properties from `string` to `DateTime` for proper
  serialization and direct access
- Created `TaxjarApiOptions` class for configurable options
- Added `AddTaxjar` extension method to register Taxjar with the service
  collection
- Replaced RestSharp with `HttpClientFactory` to prevent port exhaustion
  and outdated DNS, and to leverage Polly for resilience
- Removed all synchronous methods to embrace asynchronous operations in
  HttpClient
- Changed method signatures to use strongly typed records instead of
  anonymous objects
- Removed calls to `GetProperty()` and `GetValue()`

CHANGELOG.md

@@ -7,6 +7,14 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ## [Unreleased]
 
+## [5.0.0] - 2024-06-27
+- Replaced RestSharp with Microsoft.Extensions.Http
+- Replaced Newtonsoft.Json with System.Text.Json
+- Added ITaxjarApi interface
+- Refactor tests
+- Remove synchronous methods
+- Update target framework to netstandard2.1
+
 ## [4.0.0] - 2023-11-07
 - Update RestSharp to version 108.0.3
 - Update Newtonsoft.Json to version 13.0.3