Add support for .NET 9.0

[AdmiringWorm]

When we bump the Cake.Core dependency to 5.0.0 we should also add support for .NET 9.0, as that is what Cake 5.x supports.

[cake-contrib-bot]

🎉 This issue has been resolved in version 5.0.0 🎉

The release can be downloaded from:

• GitHub release
• NuGet.org
• Within Cake: `#addin "nuget:?package=Cake.Codecov&version=5.0.0"

Your friendly GitReleaseManager bot 📦🚀