Merged PR 43022: Update STJ to 8.0.5 in internal/release/dev17.8

#### AI description  (iteration 1)
#### PR Classification
Dependency update.

#### PR Summary
This pull request updates the `System.Text.Json` dependency to version 8.0.5 and modifies related configuration files.
- Updated `System.Text.Json` version to 8.0.5 in `Microsoft.CodeAnalysis.Workspaces.MSBuild.BuildHost.csproj`.
- Added `SystemTextJsonFixedVersion` as 8.0.5 in `eng/Versions.props`.
- Updated `darc-int-dotnet-runtime` package source URL in `NuGet.config`.

NuGet.config

@@ -19,6 +19,7 @@
     <add key="vs-impl" value="https://pkgs.dev.azure.com/azure-public/vside/_packaging/vs-impl/nuget/v3/index.json" />
     <add key="vs-impl-archived" value="https://pkgs.dev.azure.com/azure-public/vside/_packaging/vs-impl-archived/nuget/v3/index.json" />
     <add key="vs-buildservices" value="https://pkgs.dev.azure.com/azure-public/vside/_packaging/vs-buildservices/nuget/v3/index.json" />
+     <add key="darc-int-dotnet-runtime" value="https://pkgs.dev.azure.com/dnceng/internal/_packaging/darc-int-dotnet-runtime-81cabf28/nuget/v3/index.json" />
   </packageSources>
   <disabledPackageSources>
     <clear />

eng/Versions.props

@@ -270,7 +270,7 @@
     <SystemTextEncodingExtensionsVersion>4.3.0</SystemTextEncodingExtensionsVersion>
     <!-- Note: When updating SystemTextJsonVersion ensure that the version is no higher than what is used by MSBuild. -->
     <SystemTextJsonVersion>7.0.3</SystemTextJsonVersion>
-    <SystemTextJsonFixedVersion>8.0.4</SystemTextJsonFixedVersion>
+    <SystemTextJsonFixedVersion>8.0.5</SystemTextJsonFixedVersion>
     <SystemThreadingChannelsVersion>7.0.0</SystemThreadingChannelsVersion>
     <SystemThreadingTasksDataflowVersion>7.0.0</SystemThreadingTasksDataflowVersion>
     <!-- We need System.ValueTuple assembly version at least 4.0.3.0 on net47 to make F5 work against Dev15 - see https://github.com/dotnet/roslyn/issues/29705 -->