Upgrade Flurl to v4.0.2

[Riges]

Summary:
This pull request upgrades the Flurl library used in the project and includes necessary adjustments to ensure compatibility with the new version.
Because Flurl v4 move to System.text.Json we need to change here the serializer.

Changes:

1. Flurl Library Upgrade:

   • Updated the Flurl library to the latest version.
   • Modified the code to accommodate breaking changes and new features introduced in the updated Flurl library.

2. Code Adjustments:

   • Updated API calls to use the new Flurl syntax and methods.
   • Ensured that all HTTP requests and responses are handled correctly with the actual Netatmo API.

3. Testing:

   • Verified that all existing unit tests pass with the upgraded Flurl library.
   • Added new tests where necessary to cover changes in the HTTP request/response handling.

Files Modified:

• tests/TestApp/Program.cs
• tests/Netatmo.Tests/EnergyClientTest.cs

Notes:

• This upgrade aims to leverage the improvements and bug fixes in the latest version of Flurl.
• Ensure that the application is thoroughly tested to confirm that the upgrade does not introduce any regressions.