Fix failing CI build workflow

[Copilot]

Summary

CI builds were failing on ubuntu-latest runners with mono: not found, exit code 127.

Root cause

The nuget restore $SOLUTION step used nuget.exe (installed via NuGet/setup-nuget), which requires Mono on Linux. GitHub-hosted ubuntu-latest runners no longer ship Mono by default.

Fix

All projects (Matterhook.NET.MatterhookClient, Matterhook.NET.MatterhookClient.Tests, ManualTests) are SDK-style with PackageReference (no packages.config), so Mono/nuget.exe isn't required at all:

• Removed the NuGet/setup-nuget step and nuget restore; replaced with dotnet restore $SOLUTION
• Replaced nuget push with dotnet nuget push in the Publish step
• Bumped actions/checkout@v2 → v4 and actions/setup-dotnet@v1 → v4 to drop deprecated Node 20 runtime warnings

Not included

Did not add a dependabot.yml config, since another agent is reportedly already working on that separately.

[PromoFaux]

@copilot please comment out the part that uploads to nuget for the time being - I need to dig about and find my account details! (Or maybe just use github to host it.. not sure how that works)

[Copilot]

@copilot please comment out the part that uploads to nuget for the time being - I need to dig about and find my account details! (Or maybe just use github to host it.. not sure how that works)

Commented out the Publish step in 9a2f930 - CI will now just build and test on PRs/tags, no package pushes until you're ready.