Fix System.Text.Json versions

[sebastienros]

Use LTS version (8.0.5) when the dependency is required:

• netstandard2.0 required the STJ package
• net6.0 doesn't have JsonNamingPolicy so the package is referenced

[Copilot]

Pull Request Overview

This PR adjusts dependency management for System.Text.Json by updating conditional compilation directives and test expectations to use the LTS version (8.0.5) and support different .NET target frameworks.

• Modified using directives and preprocessor conditions in MemberNameStrategies.cs.
• Updated the test assertions in TemplateTests.cs to reflect the new behavior for NET8_0_OR_GREATER.
• Removed a redundant using directive in TemplateOptions.cs.

Reviewed Changes

Copilot reviewed 3 out of 6 changed files in this pull request and generated no comments.

File	Description
Fluid/MemberNameStrategies.cs	Updated preprocessor directives and modified member naming strategy methods.
Fluid.Tests/TemplateTests.cs	Revised expected outputs for member naming based on the new .NET conditions.
Fluid/TemplateOptions.cs	Removed an unnecessary using directive for System.Text.Json.

Files not reviewed (3)

• Directory.Packages.props: Language not supported
• Fluid/Fluid.csproj: Language not supported
• Versions.props: Language not supported

Comments suppressed due to low confidence (1)

Fluid/MemberNameStrategies.cs:14

• The fields 'CamelCase' and 'SnakeCase' are declared but not assigned any values in the current configuration, which may lead to null reference issues at runtime. Consider assigning them to the appropriate renaming methods (e.g., 'RenameCamelCase' and 'RenameSnakeCase') in all target framework conditions.

public static readonly MemberNameStrategy CamelCase;