Backport `HashedWheelTimerFix` to .NET Standard 2.0

[Aaronontheweb]

Changes

Port https://github.com/akkadotnet/akka.net/pull/7174 to .NET Standard 2.0

Checklist

For significant changes, please ensure that the following have been completed (delete if not relevant):

• [x] This change follows the Akka.NET API Compatibility Guidelines.
• [x] This change follows the Akka.NET Wire Compatibility Guidelines.
• [x] I have reviewed my own pull request.
• [x] Design discussion issue #7174

[Aaronontheweb]

Per my conversation with @Arkatufus in Discord, we're going to push this out to v1.5.21 until after we've seen how #7174 performs in live environments