Update getting started C# code, remove randomize

[raulsntos]

• Sync C# code with GDScript.
• Remove randomize calls.
  • This fixes one point of https://github.com/godotengine/godot-docs/issues/6744.
• Rename members that have been renamed in Godot's C# API for 4.0.
  • Fixes https://github.com/godotengine/godot-docs/issues/6878.
• Change delta parameter type to double in C#.
• Follow our C# code style more closely.
• Other minor code fixes.