Use multi targets projects for coverlet.collector, coverlet.msbuild.tasks packages

[Bertk]

This pull request updates the project to support .NET 8 and 9, modernizes dependencies, and refines the build and testing process. The most important changes include updating target frameworks, revising dependency versions, improving documentation, and enhancing build and test configurations.

Note

This PR should be merged after PR #1733.

[Copilot]

Pull Request Overview

This pull request modernizes the project to support .NET 8 and 9 as the target frameworks, replacing the previous .NET 6.0 support. The most significant changes include updating target frameworks across all projects, migrating from Newtonsoft.Json to System.Text.Json, consolidating build configurations, and improving package management for multi-targeting scenarios.

Key Changes:

• Updated target frameworks from .NET 6.0 to .NET 8.0 (minimum) and .NET 9.0 (current)
• Migrated JSON handling from Newtonsoft.Json to System.Text.Json throughout the codebase
• Restructured MSBuild tasks and collector packages to support multi-targeting with proper NuGet package layout

Reviewed Changes

Copilot reviewed 42 out of 44 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
test/coverlet.tests.utils/TestUtils.cs	Updated framework detection logic to support .NET 8.0, 9.0, and 10.0, simplified test results path
test/coverlet.integration.tests/Msbuild.cs	Updated test framework references from net6.0 to net8.0/net9.0
src/coverlet.msbuild.tasks/coverlet.msbuild.tasks.csproj	Added multi-targeting support (netstandard2.0 and NetMinimum) with updated package layout
src/coverlet.core/Reporters/JsonReporter.cs	Migrated from Newtonsoft.Json to System.Text.Json with proper serialization options
src/coverlet.core/Coverage.cs	Replaced Newtonsoft.Json dependencies with System.Text.Json throughout coverage processing
global.json	Updated .NET SDK version from 8.0.407 to 9.0.302
Directory.Packages.props	Updated package versions and removed Newtonsoft.Json dependency

Comments suppressed due to low confidence (2)

src/coverlet.core/Instrumentation/CecilAssemblyResolver.cs:325

• The variable name 'runtimeoptionselement' is inconsistent with the naming pattern used elsewhere in the method. It should be 'includedFrameworksElement' for clarity and consistency.

      if (runtimeOptionsElement.TryGetProperty("includedFrameworks", out JsonElement runtimeoptionselement))

test/coverlet.integration.tests/DeterministicBuild.cs:22

• [nitpick] The field name '_testProjectTfms' uses an abbreviation 'Tfms'. Consider using the full name '_testProjectTargetFrameworks' for better clarity.

    private string[] _testProjectTfms = [];

[Frulfump]

Since .NET 10 RC 1 is expected in September and GA in November does it make sense to start looking at that as well in case there are adaptations you would like to be present for this PR as well?