Bump MongoDB.Driver from 2.20.0 to 2.21.0

Bumps [MongoDB.Driver](https://github.com/mongodb/mongo-csharp-driver) from 2.20.0 to 2.21.0.
- [Release notes](https://github.com/mongodb/mongo-csharp-driver/releases)
- [Commits](mongodb/mongo-csharp-driver@v2.20.0...v2.21.0)

---
updated-dependencies:
- dependency-name: MongoDB.Driver
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

src/Directory.Packages.props

@@ -7,7 +7,7 @@
     <PackageVersion Include="Akka.Persistence.Hosting" Version="$(AkkaHostingVersion) " />
     <PackageVersion Include="Akka.Persistence.Query" Version="$(AkkaVersion)" />
     <PackageVersion Include="Akka.Streams" Version="$(AkkaVersion) " />
-    <PackageVersion Include="MongoDB.Driver" Version="2.20.0" />
+    <PackageVersion Include="MongoDB.Driver" Version="2.21.0" />
   </ItemGroup>
   <!-- Test dependencies -->
   <ItemGroup>