Update to .NET8 and minimal API

[eimantas]

• remove Startup.cs
• use file scoped namespaces
• remove unused imports
• using statements converted to declarations
• rename fields to C# convention

I went bananas on JetBrains Rider's auto-fixes and refactorings and bumped .NET to version 8.0. I saw there's an arm template, but I couldn't find any stack version references there. Also I did not touch the PassValidator.Test project as it was not included in the solution.

Let me know if this is something you could use and/or I should split this into smaller PRs.

Great tool! It helped me a lot in making my side gig where people can convert the plastic cards to AppleWallet passes for over 60 retailers ❤️