Add .NET 3.1 SDK to release.yaml setup-dotnet steps that run build

[Copilot]

release.yaml was installing only .NET SDKs 5.0–10.0 before running dotnet restore/dotnet build, but Wolfgang.Etl.Abstractions.csproj targets netcoreapp3.1 among its TFMs — causing the build to fail without the 3.1 SDK present.

Description

Added 3.1.x to the dotnet-version list in both setup-dotnet steps that precede build operations:

• validate-release job — runs full dotnet restore + dotnet build --configuration Release
• pack-and-validate job — runs dotnet restore + dotnet build before packing NuGet packages

The publish-nuget job's setup-dotnet step is unchanged; it only pushes pre-built packages and runs no build steps.

- name: Setup .NET
  uses: actions/setup-dotnet@v4
  with:
    dotnet-version: |
      3.1.x   # ← added; required by netcoreapp3.1 TFM in the main project
      5.0.x
      6.0.x
      ...
      10.0.x

Type of change

• Bug fix

How Has This Been Tested?

• Workflow YAML validated via CodeQL (0 alerts)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Screenshots (if applicable)

N/A

Additional context

Addresses feedback from #58 review. The same pattern (netcoreapp3.1 TFM without a matching SDK) affects pr.yaml and other workflows but is out of scope for this PR.

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

[Copilot]

Pull request overview

This PR fixes a build failure in release.yaml by adding the .NET 3.1 SDK to the two setup-dotnet steps that precede actual build operations. The project targets netcoreapp3.1 as one of its TFMs, but the release workflow was only installing .NET 5.0–10.0, which caused dotnet restore/dotnet build to fail when attempting to resolve that target.

Changes:

• 3.1.x added to the dotnet-version list in the validate-release job's setup-dotnet step (which runs dotnet restore and dotnet build --configuration Release)
• 3.1.x added to the dotnet-version list in the pack-and-validate job's setup-dotnet step (which also runs dotnet restore and dotnet build before packing)

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.