Update to .NET 9 and C# 13

[laurentkempe]

This pull request includes several changes to improve code formatting, update dependencies, and enhance functionality. The most important changes include updating the target framework, modifying the ObservableFileSystemWatcher class, and updating package references.

Framework and Dependencies Updates:

• DynamicRun/DynamicRun.csproj: Updated the target framework to net9.0 and language version to 13.
• DynamicRun/DynamicRun.csproj: Updated package references to newer versions of Microsoft.CodeAnalysis.Compilers, Microsoft.CodeAnalysis.CSharp, and System.Reactive.

Code Enhancements:

• DynamicRun/Builder/ObservableFileSystemWatcher.cs: Refactored the ObservableFileSystemWatcher class to improve readability and added the Changed property back to the class. [1] [2] [3]

Code Formatting:

• DynamicRun/Builder/Compiler.cs: Improved code formatting in the Compile and GenerateCode methods for better readability. [1] [2]
• DynamicRun/Builder/SimpleUnloadableAssemblyLoadContext.cs: Reformatted the Load method for better readability.
• DynamicRun/Program.cs: Reformatted the Program.cs file for better readability and consistency. [1] [2]

Documentation:

• README.md: Updated the README to include .NET 9 in the list of supported frameworks.

Copilot reviewed 5 out of 6 changed files in this pull request and generated no suggestions.

Files not reviewed (1)

• DynamicRun/DynamicRun.csproj: Language not supported