Bump AkkaVersion from 1.5.5 to 1.5.6 (#288)

Bumps `AkkaVersion` from 1.5.5 to 1.5.6.

Updates `Akka` from 1.5.5 to 1.5.6
- [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@1.5.5...1.5.6)

Updates `Akka.TestKit.Xunit2` from 1.5.5 to 1.5.6
- [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@1.5.5...1.5.6)

---
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
...

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

src/common.props

@@ -13,7 +13,7 @@ upgraded to Quartz.NET 3.6.2</PackageReleaseNotes>
     </NoWarn>
   </PropertyGroup>
   <PropertyGroup>
-    <AkkaVersion>1.5.5</AkkaVersion>
+    <AkkaVersion>1.5.6</AkkaVersion>
     <QuartzVersion>3.6.2</QuartzVersion>
     <XunitVersion>2.4.2</XunitVersion>
     <XunitRunnerVersion>2.4.5</XunitRunnerVersion>