feat: Add support for .NET8

[filzrev]

What's included in this PR

• Update CI build to use .NET 8 SDK.
• Update dotnet-sign tool version.
• Update Microsoft.CodeAnalysis* packages to latest preview version (.NET 8 RTM version is not released yet)
• Update following NuGet packages to .NET 8 version
  • Microsoft.Build
  • System.Collections.Immutable
  • System.Composition
• Change TargetFrameworks to net6.0;net7.0;net8.0
  (it might be possible to drop .NET7 support. Because .NET 7 support lifecycle ends at May 14, 2024)

Backlog

• It failed to run MetadataCommandTest at Docfx.Tests project when running on .NET 7 runtime. (with .NET 8 SDK)
• Following packages needs to be updated to .NET 8 RTM version.
  • Microsoft.CodeAnalysis*
  • NuGet.Frameworks
• Add C# 12 specific tests (e,g, C# 12 `ref readonly` parameters support #9339)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a14aa4c) 71.37% compared to head (7a48627) 71.39%.
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9424      +/-   ##
==========================================
+ Coverage   71.37%   71.39%   +0.01%     
==========================================
  Files         561      561              
  Lines       23961    23961              
  Branches     4163     4163              
==========================================
+ Hits        17102    17106       +4     
+ Misses       5764     5762       -2     
+ Partials     1095     1093       -2     

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[filzrev]

It failed to run MetadataCommandTest at Docfx.Tests project when running on .NET 7 runtime. (with .NET 8 SDK)

This problems is occurred on my local environment (Windows) only.
And it's not occurred on CI environment. (I've tested by temporary enable .NET 7 CI tests for Windows/macOS).

It can be resolved by this commit (c9bb072)
It seems Directory.Build.props settings are applied for tests assets project.
And both TargetFramework and TargetFrameworks are specified.
So I've changed TargetFramework setting to TargetFrameworks to use project-side settings.

[yufeih]

Looks great! Thank you @filzrev !

[filzrev]

I've removed Draft from PR.

Remaining issues
Following issues should be resolved by another PR.

• When .NET 8 RTM version released. Update following packages by dependabot.
  • Microsoft.CodeAnalysis*
  • NuGet.Frameworks
• Add C# 12 features tests. (If required)