build(deps): bump System.Text.Json in /csharp/PhoneNumbers.Extensions

Bumps System.Text.Json from 7.0.4 to 8.0.4.

---
updated-dependencies:
- dependency-name: System.Text.Json
  dependency-type: direct:production
...

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

csharp/PhoneNumbers.Extensions/PhoneNumbers.Extensions.csproj

@@ -36,7 +36,7 @@
     </ItemGroup>
 
     <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0'">
-      <PackageReference Include="System.Text.Json" Version="7.0.4" />
+      <PackageReference Include="System.Text.Json" Version="8.0.4" />
     </ItemGroup>
 
 </Project>