Bump Nethereum.HdWallet and Nethereum.Signer

Bumps [Nethereum.HdWallet](https://github.com/Nethereum/Nethereum) and [Nethereum.Signer](https://github.com/Nethereum/Nethereum). These dependencies needed to be updated together.

Updates `Nethereum.HdWallet` from 4.14.0 to 4.21.4
- [Release notes](https://github.com/Nethereum/Nethereum/releases)
- [Commits](https://github.com/Nethereum/Nethereum/commits)

Updates `Nethereum.Signer` from 4.14.0 to 4.21.4
- [Release notes](https://github.com/Nethereum/Nethereum/releases)
- [Commits](https://github.com/Nethereum/Nethereum/commits)

---
updated-dependencies:
- dependency-name: Nethereum.HdWallet
  dependency-type: direct:production
  update-type: version-update:semver-minor
- dependency-name: Nethereum.Signer
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Directory.Packages.props

@@ -4,7 +4,7 @@
   </PropertyGroup>
   <ItemGroup>
     <PackageVersion Include="Microsoft.IdentityModel.Tokens" Version="8.0.1" />
-    <PackageVersion Include="Nethereum.HdWallet" Version="4.14.0" />
+    <PackageVersion Include="Nethereum.HdWallet" Version="4.21.4" />
     <PackageVersion Include="Newtonsoft.Json" Version="13.0.3" />
     <PackageVersion Include="Portable.BouncyCastle" Version="1.9.0" />
     <PackageVersion Include="Websocket.Client" Version="4.7.0" />
@@ -14,6 +14,6 @@
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
     </PackageVersion>
-    <PackageVersion Include="Nethereum.Signer" Version="4.14.0" />
+    <PackageVersion Include="Nethereum.Signer" Version="4.21.4" />
   </ItemGroup>
 </Project>