Add .NET 6.x setup step to dotnetcore.yml workflow

[Copilot]

Summary

This PR adds the missing .NET 6.x SDK setup step to the GitHub Actions workflow in .github/workflows/dotnetcore.yml.

Problem

The workflow already includes a test step that runs tests against the net6.0 target framework:

- name: Test with .NET 6.0.x
  run: dotnet test Microsoft.Identity.Web.sln -f net6.0 -v m -p:FROM_GITHUB_ACTION=true --configuration Release --filter "..."

However, the workflow was missing the corresponding SDK setup step. While .NET 9.0.x was explicitly configured:

- name: Setup .NET 9.0.x
  uses: actions/setup-dotnet@v4
  with:
    dotnet-version: 9.0.x

There was no equivalent setup for .NET 6.x, which could cause the .NET 6.0 tests to fail if the SDK wasn't available in the GitHub Actions runner environment.

Solution

Added the .NET 6.x SDK setup step before the existing .NET 9.0.x setup:

- name: Setup .NET 6.x
  uses: actions/setup-dotnet@v4
  with:
    dotnet-version: '6.x'

This ensures that both .NET 6.0 and 9.0 SDKs are available for building and testing, which aligns with the project's multi-targeting configuration that includes net6.0 as one of the target frameworks in Directory.Build.props.

Validation

• ✅ YAML syntax validation passed
• ✅ CodeQL security analysis passed (0 alerts)
• ✅ Change follows the same pattern used in other workflow files
• ✅ Project already supports net6.0 target framework

Fixes #[issue_number]

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add net6.0 to dotnetcore.yml</issue_title>
<issue_description>In .github/workflows/dotnetcore.yml, add net6.0:

    - name: Setup .NET 6.x
      uses: actions/setup-dotnet@v4
      with:
        dotnet-version: '6.x'
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

Fixes #3488

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[jmprieur]

LGTM
Thanks