refactor: migrate to slnx format

[vbreuss]

This PR migrates the solution file format from the traditional .sln format to the newer XML-based .slnx format introduced in Visual Studio 2022.

Key Changes:

• Replaced the legacy System.IO.Abstractions.sln file with a new System.IO.Abstractions.slnx file
• Updated the NUKE build configuration to reference the new .slnx file
• Preserved all project references and folder organization in the new format

[Copilot]

Pull request overview

This PR migrates the solution file format from the traditional .sln format to the newer XML-based .slnx format introduced in Visual Studio 2022.

Changes:

• Replaced the legacy System.IO.Abstractions.sln file with a new System.IO.Abstractions.slnx file
• Updated the NUKE build configuration to reference the new .slnx file
• Preserved all project references and folder organization in the new format

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
System.IO.Abstractions.slnx	New XML-based solution file containing all projects organized into folders (Benchmarks, Meta, Pipeline, Tests)
System.IO.Abstractions.sln	Removed the legacy Visual Studio solution file
.nuke/parameters.json	Updated solution reference from .sln to .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

[github-actions]

Test Results

    31 files  ±0      31 suites  ±0   21s ⏱️ +7s
 1 151 tests ±0   1 150 ✅ ±0    1 💤 ±0  0 ❌ ±0 
11 057 runs  ±0  10 218 ✅ ±0  839 💤 ±0  0 ❌ ±0 

Results for commit 01e31b8. ± Comparison against base commit c9c34d0.