Bump AkkaVersion from 1.4.20 to 1.4.21 (#139)

Bumps `AkkaVersion` from 1.4.20 to 1.4.21.

Updates `Akka` from 1.4.20 to 1.4.21
- [Release notes](https://github.com/akkadotnet/akka.net/releases)
- [Changelog](https://github.com/akkadotnet/akka.net/blob/dev/RELEASE_NOTES.md)
- [Commits](akkadotnet/akka.net@1.4.20...1.4.21)

Updates `Akka.TestKit.Xunit2` from 1.4.20 to 1.4.21
- [Release notes](https://github.com/akkadotnet/akka.net/releases)
- [Changelog](https://github.com/akkadotnet/akka.net/blob/dev/RELEASE_NOTES.md)
- [Commits](akkadotnet/akka.net@1.4.20...1.4.21)

Updates `Akka.Cluster` from 1.4.20 to 1.4.21
- [Release notes](https://github.com/akkadotnet/akka.net/releases)
- [Changelog](https://github.com/akkadotnet/akka.net/blob/dev/RELEASE_NOTES.md)
- [Commits](akkadotnet/akka.net@1.4.20...1.4.21)

---
updated-dependencies:
- dependency-name: Akka
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: Akka.TestKit.Xunit2
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: Akka.Cluster
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

src/common.props

@@ -14,7 +14,7 @@
     <XunitVersion>2.4.1</XunitVersion>
     <TestSdkVersion>16.10.0</TestSdkVersion>
     <NBenchVersion>2.0.1</NBenchVersion>
-    <AkkaVersion>1.4.20</AkkaVersion>
+    <AkkaVersion>1.4.21</AkkaVersion>
     <NetCoreTestVersion>netcoreapp3.1</NetCoreTestVersion>
     <NetFrameworkTestVersion>net461</NetFrameworkTestVersion>
     <NetStandardLibVersion>netstandard2.0</NetStandardLibVersion>