Freshness updates for STJ source generation

[gewarren]

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docs/standard/serialization/system-text-json/source-generation.md	How to use source generation in System.Text.Json

[Copilot]

Pull Request Overview

This PR updates the System.Text.Json source generation documentation for freshness, removing outdated version-specific information and modernizing code examples to use current .NET patterns and style guidelines.

Key changes:

• Updated documentation date and removed .NET version requirements that are no longer relevant
• Modernized code examples with collection expressions and simplified API calls
• Consolidated and simplified explanatory text following Markdown style guidelines

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

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File	Description
docs/standard/serialization/system-text-json/source-generation.md	Updated metadata date, removed version-specific language, simplified text to follow style guidelines (using "might" instead of "may", "run time" instead of "runtime"), and clarified mode names
docs/standard/serialization/system-text-json/snippets/source-generation/csharp/SystemTextJsonSamples.csproj	Updated target framework from net9.0 to net10.0
docs/standard/serialization/system-text-json/snippets/source-generation/csharp/SerializeOnlyWithOptions.cs	Reformatted object initializer to use multiple lines for better readability
docs/standard/serialization/system-text-json/snippets/source-generation/csharp/Program.cs	Removed unused imports and deleted reference to removed JsonSerializerOptionsExample
docs/standard/serialization/system-text-json/snippets/source-generation/csharp/ObjectProperties.cs	Modernized collection initialization using collection expression syntax and removed unused import
docs/standard/serialization/system-text-json/snippets/source-generation/csharp/MetadataOnlyNoOptions.cs	Removed unnecessary null-forgiving operators
docs/standard/serialization/system-text-json/snippets/source-generation/csharp/JsonSerializerOptionsExample.cs	Deleted entire file (no longer referenced in documentation)
docs/standard/serialization/system-text-json/snippets/source-generation/csharp/BothModesNoOptions.cs	Simplified context class formatting, leveraged type inference in generic method calls, and streamlined serialization/deserialization examples