Migrate Cecilifier to C#13/.NET 9 🚀

[adrianoc]

• Check staging/production about compatibility with .NET 9.0 and apply changes if needed.
• Change projects to target C# 13
• Change Cecilifier to target C# 13
• Report C# 13 features as not supported
• Search (text) for .NET 8.0 in the project folder to see if there are any
  references to these and act accordingly