Consider using new TimeProvider instead of ITimer and ITimerFactory

[joperezr]

Now that TimeProvider APIs have been added to .NET 8.0, we should consider if we want to replace some internal types that are used by Yarp. For instance, we should be able to replace

https://github.com/microsoft/reverse-proxy/blob/b6f00851fc1032706c275b414244a95ef70d3a26/src/ReverseProxy/Utilities/ITimerFactory.cs#L8-L11

with The new TimeProvider.CreateTimer that was introduced:

https://github.com/dotnet/runtime/blob/ce4857970dff1e987a3991238c8b32e8eae3cab7/src/libraries/Common/src/System/TimeProvider.cs#L137

cc: @tarekgh @Tratcher

[MihaZupan]

Seems reasonable to replace YARP's current IClock/ITimer/ITimerFactory once TimeProvider is finalized and down-level packages ship.
Given we don't want to drag IClock (public API) along forever, it'd make sense to make the transition and drop IClock in a major version.

[Tratcher]

We'll need to come up with a transition plan for these things. Ideally there's one version where both are available, and one may be marked obsolete.