Bump AkkaVersion from 1.4.20 to 1.4.21 (#227)

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.Serialization.Hyperion` 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)

---
updated-dependencies:
- dependency-name: Akka
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: Akka.Serialization.Hyperion
  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
...

Signed-off-by: dependabot[bot] <[email protected]>

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

Author: dependabot[bot]

src/common.props

@@ -18,7 +18,7 @@
     <NoWarn>$(NoWarn);CS1591</NoWarn>
   </PropertyGroup>
   <PropertyGroup>
-    <AkkaVersion>1.4.20</AkkaVersion>
+    <AkkaVersion>1.4.21</AkkaVersion>
 
     <FluentAssertionsVersion>5.10.3</FluentAssertionsVersion>
     <XunitVersion>2.4.1</XunitVersion>