Update GitHub Actions workflows to use .NET 9 instead of .NET Core 3.1.301

[Copilot]

Updates the GitHub Actions workflows to use .NET 9.0.x instead of the outdated .NET Core 3.1.301, ensuring compatibility with the project's current .NET 9 target framework.

Changes Made

• Updated dotnet-version from 3.1.301 to 9.0.x in .github/workflows/build-and-test.yml
• Updated dotnet-version from 3.1.301 to 9.0.x in .github/workflows/publish-packages.yml

Context

The project files already target .NET 9 (net9.0):

• src/Html2md/Html2md.csproj
• src/Html2md.Core/Html2md.Core.csproj
• test/Html2md.Tests.Unit/Html2md.Tests.Unit.csproj

However, the GitHub Actions workflows were still configured to use .NET Core 3.1.301, causing a mismatch that would prevent the CI/CD pipeline from building and testing the project correctly.

Verification

• Both workflow files maintain valid YAML syntax
• Changes are minimal and surgical (only 2 lines changed across 2 files)
• The 9.0.x version specifier ensures the latest .NET 9 patch version is used

This pull request was created as a result of the following prompt from Copilot chat.

Update the GitHub Actions workflow in .github/workflows/build-and-test.yml to use .NET 9 instead of .NET Core 3.1.301. This includes changing the dotnet-version to 9.0.x to ensure the workflow sets up the correct .NET version for building and testing the project.

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