Add support for .NET 10

[ffernandolima]

What

Removed support for .NET 6 as it has reached the end of support;
Added support for .NET 10;
Set 'LangVersion' to 'latest'.

Checklist

References

Test & Review

Open questions / Follow-ups

[Copilot]

Pull request overview

This PR modernizes the .NET support across the project by removing support for .NET 6 (which has reached end of life) and adding support for .NET 10. The changes also update the C# language version from specific older versions (7.1, 7.3) to "latest" to leverage modern C# features.

• Removed .NET 6 from all multi-targeted projects and added .NET 10
• Updated single-target projects from .NET 8 to .NET 10
• Upgraded LangVersion from specific older versions to "latest"

Reviewed changes

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

Show a summary per file

File	Description
test/Confluent.SchemaRegistry.UnitTests/Confluent.SchemaRegistry.UnitTests.csproj	Updated target frameworks from net6.0;net8.0 to net8.0;net10.0
test/Confluent.SchemaRegistry.Serdes.UnitTests/Confluent.SchemaRegistry.Serdes.UnitTests.csproj	Updated target frameworks from net6.0;net8.0 to net8.0;net10.0
test/Confluent.SchemaRegistry.Serdes.IntegrationTests/Confluent.SchemaRegistry.Serdes.IntegrationTests.csproj	Updated target frameworks from net6.0;net8.0 to net8.0;net10.0
test/Confluent.SchemaRegistry.IntegrationTests/Confluent.SchemaRegistry.IntegrationTests.csproj	Updated target frameworks from net6.0;net8.0 to net8.0;net10.0
test/Confluent.Kafka.VerifiableClient/Confluent.Kafka.VerifiableClient.csproj	Updated target frameworks from net6.0;net8.0 to net8.0;net10.0
test/Confluent.Kafka.UnitTests/Confluent.Kafka.UnitTests.csproj	Updated target frameworks from net6.0;net8.0 to net8.0;net10.0
test/Confluent.Kafka.Transactions/Confluent.Kafka.Transactions.csproj	Updated target frameworks to net8.0;net10.0 and LangVersion to latest
test/Confluent.Kafka.TestsCommon/Confluent.Kafka.TestsCommon.csproj	Updated target frameworks from net6.0;net8.0 to net8.0;net10.0
test/Confluent.Kafka.SyncOverAsync/Confluent.Kafka.SyncOverAsync.csproj	Updated target frameworks from net6.0;net8.0 to net8.0;net10.0
test/Confluent.Kafka.IntegrationTests/Confluent.Kafka.IntegrationTests.csproj	Updated target frameworks from net6.0;net8.0 to net8.0;net10.0
test/Confluent.Kafka.Benchmark/Confluent.Kafka.Benchmark.csproj	Updated target frameworks from net6.0;net8.0 to net8.0;net10.0
src/Confluent.SchemaRegistry.Serdes.Json/Confluent.SchemaRegistry.Serdes.Json.csproj	Updated package reference conditions to include net10.0 alongside net8.0
src/ConfigGen/ConfigGen.csproj	Updated target frameworks to net8.0;net10.0 and LangVersion to latest
examples/*/*.csproj (multiple files)	Updated TargetFramework from net8.0 to net10.0 and LangVersion to latest
examples/JsonWithReferences/Program.cs	Added conditional compilation for NET8_0_OR_GREATER to handle NJsonSchema namespace differences

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