Bump AkkaVersion from 1.4.19 to 1.4.20 (#207)
Bump AkkaVersion from 1.4.19 to 1.4.20 (#207)

Bumps `AkkaVersion` from 1.4.19 to 1.4.20.

Updates `Akka.TestKit.XUnit2` from 1.4.19 to 1.4.20
- [Release notes](https://github.com/akkadotnet/akka.net/releases)
- [Changelog](https://github.com/akkadotnet/akka.net/blob/dev/RELEASE_NOTES.md)
- [Commits](https://github.com/akkadotnet/akka.net/compare/1.4.19...1.4.20)

Updates `Akka.Cluster` from 1.4.19 to 1.4.20
- [Release notes](https://github.com/akkadotnet/akka.net/releases)
- [Changelog](https://github.com/akkadotnet/akka.net/blob/dev/RELEASE_NOTES.md)
- [Commits](https://github.com/akkadotnet/akka.net/compare/1.4.19...1.4.20)

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