Add support for .NET 9

[ffernandolima]

• Removed support for .NET 6 as it has reached the end of support;
• Added support for .NET 9;
• Set 'LangVersion' to 'latest'.

[confluent-cla-assistant]

🎉 All Contributor License Agreements have been signed. Ready to merge.
✅ ffernandolima
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[Havret]

Would it make sense to add support for a version of .NET that isn't LTS?

[ffernandolima]

Would it make sense to add support for a version of .NET that isn't LTS?

@Havret, I agree with you to some extent, and I believe this is open for discussion. However, the primary objective of this PR is to remove .NET 6, as it has reached its end of support and should no longer be used in active development.

[Im5tu]

Would it make sense to add support for a version of .NET that isn't LTS?

@Havret, I agree with you to some extent, and I believe this is open for discussion. However, the primary objective of this PR is to remove .NET 6, as it has reached its end of support and should no longer be used in active development.

Could just do a multi-target so you always have current STS + LTS versions, ie: net8/net9.

[ffernandolima]

Could just do a multi-target so you always have current STS + LTS versions, ie: net8/net9.

@Im5tu, that was exactly what I did through this PR. Projects released as Nuget packages support multi-target.

[ffernandolima]

Hi @anchitj @rayokota, would it be possible to get this PR reviewed? Do you guys see any problems related to .NET 8 and .NET 9 multi-targeting and the removal of .NET 6? I'd appreciate it to get your opinion/point of view here.

[lgcmotta]

Would it make sense to add support for a version of .NET that isn't LTS?

I think yes, it makes sense. Each yearly release, whether STS or LTS, includes valuable performance improvements, new features, and bug fixes. If you only upgrade every two years, you risk missing out on incremental enhancements. Another point is: that by updating more frequently, you typically face smaller, more manageable changes rather than a large, potentially disruptive jump.
Although you’re unlikely to encounter major breaking changes on the language or packages API, there could still be under-the-hood modifications that introduce subtle issues if you skip multiple versions.

[Copilot]

Pull Request Overview

This pull request updates the project to support .NET 9 while removing support for the end-of-life .NET 6 framework. The changes modernize the language version settings and update conditional compilation directives to accommodate the new framework.

• Updated target frameworks from .NET 6 and .NET 8 to .NET 8 and .NET 9 across all projects
• Updated LangVersion from specific versions (7.1, 7.3) to 'latest' for modernized C# features
• Modified conditional compilation for NJsonSchema packages to support .NET 9

Reviewed Changes

Copilot reviewed 34 out of 34 changed files in this pull request and generated no comments.

File	Description
Test projects	Updated TargetFrameworks from net6.0;net8.0 to net8.0;net9.0
Source projects	Updated framework targeting and package conditions for .NET 9 support
Example projects	Migrated from net6.0 to net9.0 and updated LangVersion to latest
JsonWithReferences example	Added conditional compilation for NJsonSchema compatibility

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