Migrate CI/CD from Azure DevOps to GitHub Actions

[poxet]

Summary

• Add .github/workflows/build.yml mirroring the Tharga.Crawler reference, customized for this repo: MAJOR_MINOR=1.14, packs only Tharga.Test.Toolkit/Tharga.Test.Toolkit.csproj, no test filter (no test projects in this repo).
• Remove obsolete azure-pipelines.yml and buildnumber.yml; drop them from Solution Items in Tharga.Test.sln.
• Implements the pending request "Tharga.Test" under "GitHub Actions CI/CD" in the central Requests notes.

Manual follow-ups (cannot be done from a PR)

• Configure NUGET_API_KEY secret in GitHub repo settings
• Configure release and prerelease environments
• Disable the old Azure DevOps build pipeline in the AzDo UI
• After merge: delete the develop branch (no longer used under the GHA branching strategy)

Test plan

• CI workflow runs green on this PR (build, security, prerelease jobs)
• After merge: stable release 1.14.8 is published to NuGet via the workflow and a GitHub Release is created

[github-advanced-security]

You are seeing this message because GitHub Code Scanning has recently been set up for this repository, or this pull request contains the workflow file for the Code Scanning tool.

What Enabling Code Scanning Means:

• The 'Security' tab will display more code scanning analysis results (e.g., for the default branch).
• Depending on your configuration and choice of analysis tool, future pull requests will be annotated with code scanning analysis results.
• You will be able to see the analysis results for the pull request's branch on this overview once the scans have completed and the checks have passed.

For more information about GitHub Code Scanning, check out the documentation.

[github-actions]

Released as v1.14.8 — https://github.com/Tharga/Test/releases/tag/1.14.8