chore!: Dotnet 8

[Soap-141]

GitHub Issue: #

Proposed Changes

• Bug fix
• Feature
• Code style update (formatting)
• Refactoring (no functional changes, no api changes)
• Build or CI related changes
• Documentation content changes
• Other, please describe:

What is the current behavior?

Projects in the solution target .NET 6-7 and C# 10-11.

What is the new behavior?

Projects in the solution target .NET 8 and C# 12.

Impact on version

• Major (Public API was modified.)
  • Public constructs (class, struct, delegate, enum, etc.) were removed or renamed.
  • Public members were removed or renamed.
  • Public method signatures were changed or renamed.
• Minor (Public API was extended.)
  • Public constructs, members, or overloads were added.
• Patch (Public API was unchanged.)
  • A bug in behavior was fixed.
  • Documentation was changed.
• None (The library is unchanged.)
  • Only code under the build folder was changed.
  • Only code under the .github folder was changed.

Checklist

Please check that your PR fulfills the following requirements:

• Documentation has been added/updated.
• Automated Unit / Integration tests for the changes have been added/updated.
• Updated BREAKING_CHANGES.md (if you introduced a breaking change).
• Your conventional commit are aligned with the Impact on version section.

Other information

[Copilot]

Pull Request Overview

This PR updates the solution's target framework to .NET 8 and adjusts related build configuration and documentation accordingly.

• Updated DotNetVersion, UnoCheck_Version, and UnoCheck_Manifest in build/stage-build.yml for .NET 8 support.
• Added breaking changes notes in BREAKING_CHANGES.md indicating support for .NET 8 and removal of .NET 7.

Reviewed Changes

Copilot reviewed 2 out of 7 changed files in this pull request and generated no comments.

File	Description
build/stage-build.yml	Updated build configuration for .NET 8 support.
BREAKING_CHANGES.md	Documented breaking changes for framework upgrade.

Files not reviewed (5)

• src/BackButtonManager.Abstractions/BackButtonManager.Abstractions.csproj: Language not supported
• src/BackButtonManager.Tests/BackButtonManager.Tests.csproj: Language not supported
• src/BackButtonManager.Uno.WinUI/BackButtonManager.Uno.WinUI.csproj: Language not supported
• src/BackButtonManager.Uno.WinUI/winappsdk-workaround.targets: Language not supported
• src/BackButtonManager/BackButtonManager.csproj: Language not supported