Update to .NET 5 RC
C# 9 will be released next week as part of .NET 5. This merge request updates the ZILF projects to take advantage of new language features and build under the latest .NET 5 RC.
- The CI pipeline now runs on the .NET 5.0 SDK image instead of .NET Core 3.1 SDK.
- ZilfAnalyzers.Vsix was removed: it was blocking the analyzers from using .NET Core-only language features, and analyzers don't need to be built as VS packages anyway.
- JetBrains.Annotations was removed, and the annotations were migrated to .NET nullability contract attributes where applicable.