upgrade cake 5.1

[AdaskoTheBeAsT]

This pull request modernizes the build and test infrastructure for the project, updates dependencies, and cleans up code for consistency and maintainability. The main focus is on upgrading to .NET 8.0 and 9.0, updating package references, and making minor code improvements for clarity and compatibility.

Target framework and dependency updates:

• Upgraded both Cake.IIS and Cake.IIS.Tests projects to target only net8.0 and net9.0, dropping support for older frameworks like net461, netstandard2.0, and net5.0. Updated all related package references to their latest compatible versions, including Cake.Core, System.ServiceProcess.ServiceController, xunit, and others. [1] [2] [3] [4]

• Updated the .nuspec packaging to only include net8.0 and net9.0 builds, added a dependency on System.Net.Http, and included README.md in the package.

Build and configuration improvements:

• Updated the appveyor.yml configuration to use test: false instead of test: off for clarity.

• Adjusted the logic in CakeHelper.CreateWebConfig to handle only the new supported frameworks (net8.0, net9.0).

Test and sample configuration enhancements:

• Added <system.webServer><directoryBrowse ... /></system.webServer> sections to all test web.config files to explicitly set directory browsing settings for test clarity. [1] [2]

• Updated test code to use explicit null! values for nullable parameters, improving nullability handling and code clarity. [1] [2]

Code consistency and cleanup:

• Standardized usage of string.Empty instead of "" and improved null/empty checks by using string.IsNullOrEmpty (instead of String.IsNullOrEmpty). [1] [2] [3] [4] [5] [6] [7]

• Cleaned up unnecessary blank lines, improved using directives, and made constructors protected where appropriate for better code organization and encapsulation. [1] [2] [3] [4] [5] [6] [7] [8] [9]

These changes collectively update the project for modern .NET development, improve maintainability, and ensure compatibility with current dependencies.