Modernize administrator and Mono version checks

[HebaruSan]

Motivations

• CKAN has a check for whether the user is privileged and an --asroot flag to bypass this, but it's Linux-only. Windows users are just as likely to benefit from this guardrail. It's also implemented entirely within CmdLine, even though it could in principle apply more generally.
• There's a check that prints a warning if the user is on a version of Mono before 3.1.0, which duplicates platform-specific logic from Core in CmdLine, and I'm pretty sure you need a much newer version of Mono to run current CKAN
• Platform.IsMonoFourOrLater and Platform.IsOnMonoFourOrLater have been unused since Master search bar and misc GUI clean-up #3041

Changes

• Now the root user check is moved to Platform.IsAdministrator()
  • Now it has an IsWindows section that checks for administrator or system roles
  • Now the messages for the root user check are platform agnostic
  • I wanted to add an --asadmin alias for the --asroot flag, but I could not find a mechanism for that in the command line parser library, and I don't want to break existing scripts or shortcuts by changing it
• Now the duplicative Mono versioning logic from Platform.IsOnMonoFourOrLater and CmdLine.Options.CheckMonoVersion is moved to a Platform.MonoVersion field
  • Now the Mono warning is printed if they're on a version before 5.0.0 (stored in Platform.RecommendedMonoVersion), which is still pretty old but not as guaranteed ancient as 3.1.0
  • This uses a new RegexExtensions.TryMatch function to check a regex for a match and use its output within a functional-style expression

[techman83]

This is super clean, and removing unused code is always a win 🎉 thanks @HebaruSan !