Upgrade to .NET 8 and latest C#

[shibayan]

This pull request includes several changes aimed at simplifying the codebase and improving consistency by replacing new[] { ... } with [...] and refactoring class constructors. The most important changes include modifications to various sample methods, class constructors, and internal structures.

Simplification and consistency improvements:

• Sharprompt.Example/Program.cs: Replaced new[] { ... } with [...] in multiple sample methods, such as RunInputSample, RunPasswordSample, RunSelectSample, RunMultiSelectSample, and RunSelectEnumSample. [1] [2] [3]
• Sharprompt.Tests/PropertyMetadataTests.cs: Changed new[] { ... } to [...] in GetSelectItems and SelectItems properties.

Class constructor refactoring:

• Sharprompt/Forms/FormRenderer.cs: Modified the class constructor to use inline initialization for _offscreenBuffer.
• Sharprompt/Internal/Optional.cs: Refactored Optional<T> to use a primary constructor and simplified property initialization.
• Sharprompt/Symbol.cs: Updated Symbol class to use primary constructor and removed redundant fields.

Internal structure updates:

• Sharprompt/Internal/OffscreenBuffer.cs: Replaced new List<TextInfo>() with [] in _outputBuffer initialization and related methods. [1] [2] [3]
• Sharprompt/Internal/Paginator.cs: Changed _filteredItems initialization from Array.Empty<T>() to [].

Project configuration update:

• Sharprompt/Sharprompt.csproj: Updated the project to target only .NET 8.0 instead of multiple frameworks.