Bump System.Text.Json in /src/NodaTime.Serialization.SystemTextJson

Bumps System.Text.Json from 6.0.0 to 8.0.4.

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

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

src/NodaTime.Serialization.SystemTextJson/NodaTime.Serialization.SystemTextJson.csproj

@@ -19,6 +19,6 @@
 
   <ItemGroup>
     <PackageReference Include="NodaTime" Version="[3.0.0,4.0.0)" />
-    <PackageReference Include="System.Text.Json" Version="6.0.0" Condition="'$(TargetFramework)' == 'netstandard2.0'" />
+    <PackageReference Include="System.Text.Json" Version="8.0.4" Condition="'$(TargetFramework)' == 'netstandard2.0'" />
   </ItemGroup>
 </Project>