Consider using `TimeProvider` using Microsoft.Bcl.TimeProvider nuget package

[JakeYallop]

There is a comment in ISystemClock here:

FeatureManagement-Dotnet/src/Microsoft.FeatureManagement/FeatureFilters/ISystemClock.cs

Lines 9 to 18 in 31b0efc

	/// <summary>
	/// Abstracts the system clock to facilitate testing.
	/// .NET8 offers an abstract class TimeProvider. After we stop supporting .NET version less than .NET8, this ISystemClock should retire.
	/// </summary>
	internal interface ISystemClock
	{
	/// <summary>
	/// Retrieves the current system time in UTC.
	/// </summary>
	public DateTimeOffset UtcNow { get; }

However this is not correct. The runtime provides a down-level package https://www.nuget.org/packages/Microsoft.Bcl.TimeProvider/ that allows TimeProvider to be used code using older versions of the runtime, or code that needs to support .NET standard.

[zhiyuanliang-ms]

@JakeYallop Thank you very much for paying attention to the latest changes in our repo.

This was a oversight. I didn't know there is Microsoft.Bcl.TimeProvider.

I have sent out a PR to update it. #452

[zhiyuanliang-ms]

The PR #452 will reference a new package. We plan to adopt this change with the next major version bump because introducing a new package might lead to potential breaking changes.