v7.0: net8.0 only, fully async

• New: TaskRunner does not create dedicated Task, instead it uses async/await (and ThreadPool), becoming more efficient;
• BREAKING: All previous framework support is dropped, only net8.0 is supported now;
• BREAKING: Start() now throws InvalidOperationException (instead of ArgumentOutOfRangeException) when Options.FirstRunDelay is negative;
• Fix: RunStatus.LastRunTime now updated even after failed run;