Refactored Singleton Pattern examples for compatibility and thread sa…

[omarbitar97]

…fety

Improved thread safety by adding locking mechanisms in singleton implementations. Refactored to ensure compatibility with C# 4.0 by replacing interpolated strings and expression-bodied members with alternatives. Simplified and standardized singleton instance access. Ensured better encapsulation with private constructors and sealed classes where applicable. Improved readability and consistency across all Singleton Pattern example scripts.