Support .NET 8 TimeProvider

[RomanKreisel]

.NET 8 introduced the TimeProvider which finally made times in unit tests mockable, as well as rollout environments for automatic testing on different dates/times.

Unfortunately, FluentScheduler doesn't support TimeProvider, so if it's part of the architecture - you cannot get any benefit of this change yet.

Since FluentScheduler - for sure - shouldn't lose backward-compatibility with earlier versions of .NET, for sure it makes sense to use Microsoft.Bcl.TimeProvider, which supports .NET Standard 2 as well .