Apply various modernizations

[brantburnett]

Motivation

Keep up to date.

Modifications

• Drop the out-of-support .NET 6 target and add a .NET 9 target.
• Use file-scoped namespaces.
• Update various NuGet dependencies and update to xUnit v3.
• Update GitHub Actions and GitVersion.
• Switch to GitHubActionsTestLogger.
• Update to C# 13.
• Reduce code duplication by forwarding RunSync calls with no state to the methods which take state parameters. This may also reduce JIT code size in some scenarios.
• Add OverloadResolutionPriority to RunSync calls so C# 13 and later prefers the Task overload to the ValueTask overload. This simplifies usage by consumers.
• Use a strong type for work items instead of a ValueTuple.

[Copilot]

Pull Request Overview

This PR modernizes the CenterEdge.Async library by updating target frameworks, dependencies, and code to use newer .NET features. The primary goals are dropping out-of-support .NET 6, adding .NET 9 support, and improving API usability with C# 13 features.

Key changes include:

• Updated target frameworks from .NET 6 to .NET 9 and upgraded to C# 13
• Refactored AsyncHelper to reduce code duplication by forwarding simple RunSync calls to stateful methods
• Added OverloadResolutionPriority attributes to prefer Task overloads over ValueTask when both are applicable

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
OverloadResolutionPriorityAttribute.cs	Adds internal polyfill for C# 13 overload resolution priority feature
CenterEdge.Async.csproj	Updates target frameworks, language version, and package dependencies
AsyncHelper.cs	Modernizes with file-scoped namespace and reduces duplication by forwarding calls
AsyncHelperTests.cs	Updates tests to use cleaner syntax and TestContext cancellation tokens
Build/CI files	Updates GitHub Actions, GitVersion, and test runner configurations

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[brantburnett]

/merge