refactor: migrate to slnx solution format

[vbreuss]

This PR migrates the solution from the traditional .sln format to the newer .slnx (XML-based) format, which is supported in modern versions of Visual Studio and .NET tooling. The migration also updates the build pipeline to target .NET 10.0 and upgrades Nuke build tools to version 10.1.0.

Key changes

• Replaced traditional Testably.Abstractions.sln file with new XML-based Testably.Abstractions.slnx format
• Updated build pipeline target framework from .NET 8.0 to .NET 10.0
• Updated Nuke build tool dependencies to version 10.1.0

[Copilot]

Pull request overview

Migrates the repository’s Visual Studio solution from the legacy .sln format to the XML-based .slnx format, and updates build tooling configuration to point to the new solution file.

Changes:

• Added Testably.Abstractions.slnx and removed Testably.Abstractions.sln.
• Updated Nuke configuration to reference the new .slnx solution.
• Updated the Nuke build project/tooling versions (Build.csproj TFM + Nuke package versions).

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
Testably.Abstractions.slnx	New .slnx solution structure (projects + solution items) to replace the removed .sln.
Testably.Abstractions.sln	Removed legacy Visual Studio solution file.
Pipeline/Build.csproj	Updates the Nuke build project target framework to net10.0.
Directory.Packages.props	Upgrades Nuke package versions used by the build project.
.nuke/parameters.json	Points Nuke’s Solution parameter at Testably.Abstractions.slnx.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud