Remove the csharp-gui-testing and csharp-configuration skills - #25

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@v4, actions/checkout@v4, actions/setup-dotnet@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Possible null reference argument for parameter 'actual' in 'void ShouldBeStringTestExtensions.ShouldContain(string actual, string expected, Case caseSensitivity = Case.Insensitive, string? customMessage = null)'.

Possible null reference argument for parameter 'actual' in 'void ShouldBeStringTestExtensions.ShouldContain(string actual, string expected, Case caseSensitivity = Case.Insensitive, string? customMessage = null)'.