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

[Copilot]

Pull Request Overview

This PR raises the minimum .NET SDK/runtime requirement to 8.0 and swaps out all Newtonsoft.Json usages in favor of System.Text.Json across core and test projects.

• Projects and tests are retargeted and package references updated to .NET 8.0.
• JSON serialization calls are migrated from Newtonsoft.Json to System.Text.Json.
• Integration tests, docs, and config files are adjusted to reflect the new framework version and package versions.

Reviewed Changes

Copilot reviewed 24 out of 24 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
test/coverlet.integration.tests/Msbuild.cs	Retarget tests to net8.0, refactor file-path assertions.
test/coverlet.integration.tests/DotnetTool.cs	Removed stray Unicode BOM from file header.
test/coverlet.integration.tests/DeterministicBuild.cs	Refined props file creation and XDocument initialization.
test/coverlet.integration.template/nuget.config	Renamed NuGet source key from nuget to nuget.org.
test/coverlet.integration.determisticbuild/coverlet.integration.determisticbuild.csproj	Updated csproj to target net8.0.
test/coverlet.core.tests/coverlet.core.tests.csproj	Cleaned up BOM character in csproj header.
test/coverlet.core.tests.samples.netstandard/coverlet.core.tests.samples.netstandard.csproj	Bumped System.Threading.Tasks.Extensions to 4.6.3.
src/coverlet.msbuild.tasks/coverlet.msbuild.targets	Raised _CoverletSdkMinVersionWithDependencyTarget to 8.0.100.
src/coverlet.core/coverlet.core.csproj	Added net8.0 target, removed explicit VersionOverride, suppressed IDE0057.
src/coverlet.core/Reporters/JsonReporter.cs	Replaced JsonConvert with JsonSerializer and configured options.
src/coverlet.core/Instrumentation/CecilAssemblyResolver.cs	Swapped Newtonsoft.Json.Linq for System.Text.Json parsing.
src/coverlet.core/Helpers/InstrumentationHelper.cs	Scoped IDE0057 suppression around substring calls.
src/coverlet.core/Exceptions.cs	Removed [Serializable] and serialization ctor stubs.
src/coverlet.core/CoverageResult.cs	Added [JsonConstructor] and new collection expression syntax.
src/coverlet.core/Coverage.cs	Swapped Newtonsoft.Json calls for System.Text.Json APIs.
global.json	Bumped SDK version to 8.0.409.
README.md	Updated notes and requirements for .NET 8 and styled as markdown note.
Documentation/VSTestIntegration.md	Updated example paths from net6.0 to net8.0.
Directory.Packages.props	Bumped multiple package versions and removed Newtonsoft.Json.
CONTRIBUTING.md	Replaced SDK list with STS 9.0 and LTS 8.0 entries.

Comments suppressed due to low confidence (3)

CONTRIBUTING.md:7

• [nitpick] This entry is missing a markdown bullet marker (e.g., - or * ); add one to keep consistent list formatting.

STS version [.NET SDK 9.0](https://dotnet.microsoft.com/en-us/download/dotnet/9.0) for development environment

src/coverlet.core/Helpers/InstrumentationHelper.cs:431

• [nitpick] Instead of suppressing IDE0057, use the C# range operator (e.g., x[1..x.IndexOf(']')]) for clearer substring logic.

x.Substring(1, x.IndexOf(']') - 1)

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

• There's a stray BOM character at the start of this file which may cause encoding issues; remove it.

// Copyright (c) Toni Solarin-Sodara

[Copilot]

[nitpick] Unnecessary use of Path.Combine with a single argument. Call Save(propsFile) directly for readability.

Suggested change

	deterministicTestProps.Save(Path.Combine(propsFile));
	deterministicTestProps.Save(propsFile);

[Copilot]

Using GetProperty will throw if runtimeOptions is missing. Consider using TryGetProperty to handle absence gracefully.

Suggested change

	JsonElement runtimeOptionsElement = rootElement.GetProperty("runtimeOptions");
	if (!rootElement.TryGetProperty("runtimeOptions", out JsonElement runtimeOptionsElement))
	{
	throw new InvalidOperationException($"The 'runtimeOptions' property is missing in the runtime configuration file {_runtimeConfigFile}.");
	}