chore(deps): bump System.Text.Json in /src/KafkaFlow.Serializer.JsonCore

Bumps System.Text.Json from 6.0.8 to 8.0.4.

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

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

src/KafkaFlow.Serializer.JsonCore/KafkaFlow.Serializer.JsonCore.csproj

@@ -8,7 +8,7 @@
     </PropertyGroup>
 
     <ItemGroup>
-        <PackageReference Include="System.Text.Json" Version="6.0.8" />
+        <PackageReference Include="System.Text.Json" Version="8.0.4" />
     </ItemGroup>
 
     <ItemGroup>