chore: migrate CI from Cloud Build to GitHub Actions

[AndreiMihalciuc]

Why this PR?

This extracts the CI migration work from the auto-register interfaces feature branch so the pipeline changes can be reviewed and shipped independently.

How does the PR achieve that?

• adds GitHub Actions workflows for staging and production builds
• removes the legacy Cloud Build configuration files
• updates the solution reference used by the workflows
• adds NuGet.config retrieval in the workflow setup

How do I test this?

• review the GitHub Actions workflow definitions under .github/workflows/
• confirm the legacy Cloud Build files under build/ are removed
• run dotnet build Dojo.Generators.sln
• if secrets and repo variables are configured, validate the staging workflow on a branch push and the production workflow on a version tag

[dojo-wiz-connector]

Wiz Scan Summary

Scanner	Findings
Vulnerabilities	-
Sensitive Data	-
Secrets	-
IaC Misconfigurations	7
SAST Findings	-
Software Management Findings	-
Total	7

View scan details in Wiz

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[gemini-code-assist]

Code Review

This pull request removes the Google Cloud Build configuration files (cloudbuild-build-only.yaml and cloudbuild.yaml) and their references from the solution file. Additionally, it enables the GitHub Actions workflows for production and staging builds by removing the .disabled suffix from their filenames in the solution. There are no review comments, and I have no feedback to provide.