Bump Akka.TestKit.XUnit2 from 1.5.1 to 1.5.6 (#211)

Bumps [Akka.TestKit.XUnit2](https://github.com/akkadotnet/akka.net) from 1.5.1 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@1.5.1...1.5.6)

---
updated-dependencies:
- dependency-name: Akka.TestKit.XUnit2
  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>

src/Directory.Build.props

@@ -1,4 +1,4 @@
-<Project>
+<Project>
   <PropertyGroup>
     <Copyright>Copyright © 2023 Your Company</Copyright>
     <Authors>Your Authors</Authors>
@@ -18,7 +18,7 @@
 	</ItemGroup>
 
   <PropertyGroup>
-    <AkkaVersion>1.5.1</AkkaVersion>
+    <AkkaVersion>1.5.6</AkkaVersion>
     <PbmVersion>1.3.0</PbmVersion>
     <MicrosoftExtensionsVersion>7.0.1</MicrosoftExtensionsVersion>
   </PropertyGroup>