use .NET 8.0 target framework for coverlet.core and remove Newtonsoft.Json

[Bertk]

• updates the minimum requirement for .NET SDK and runtime to .NET 8.0
• replaces Newtonsoft.Json nuget packages with System.Text.Json which is shipped with .NET 8.0

related: #1713, #1762

[Copilot]

Pull Request Overview

This PR upgrades the minimum requirement for .NET SDK and runtime to .NET 8.0 and replaces Newtonsoft.Json with System.Text.Json throughout the codebase. The changes modernize the project to leverage .NET 8.0 features and the built-in JSON serialization.

Key changes include:

• Updating target frameworks from .NET 6.0 to .NET 8.0 across test and example projects
• Replacing Newtonsoft.Json with System.Text.Json in core components and package references
• Updating minimum SDK version requirements and documentation to reflect .NET 8.0 LTS support

Reviewed Changes

Copilot reviewed 45 out of 47 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/coverlet.core/coverlet.core.csproj	Multi-targets to include net8.0, removes Newtonsoft.Json dependency
src/coverlet.core/Reporters/JsonReporter.cs	Migrates from Newtonsoft.Json to System.Text.Json serialization
src/coverlet.core/Instrumentation/CecilAssemblyResolver.cs	Updates JSON parsing from JObject to JsonDocument
src/coverlet.core/Coverage.cs	Replaces Newtonsoft.Json with System.Text.Json for deserialization
test/coverlet.tests.utils/TestUtils.cs	Adds support for .NET 9.0 and 10.0 target frameworks
test/coverlet.integration.tests/Msbuild.cs	Updates test framework targets and fixes multi-framework test logic
Documentation/	Updates all documentation to reflect .NET 8.0 requirements
global.json	Updates SDK version to 8.0.413

Comments suppressed due to low confidence (1)

test/coverlet.integration.tests/Msbuild.cs:1

• The variable name 'runtimeoptionselement' is inconsistent with the naming convention used elsewhere. It should be 'runtimeOptionsElement' to match the camelCase pattern used for other similar variables in the same method.

// Copyright (c) Toni Solarin-Sodara

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