chore: remove FASTGlobal version tracking

[janechu]

Pull Request

📖 Description

Remove the FASTGlobal.versions API from the v3 branch so FAST no longer exposes runtime version tracking. This keeps the change as a pure breaking-change removal: the public API/docs are updated, the migration guidance explains the unsupported multi-version scenario, and no legacy runtime fallback is added.

🎫 Issues

• Closes Remove versions in FASTGlobal #7278

👩‍💻 Reviewer Notes

This revision intentionally keeps the work to API and documentation removal only. Reviewers can focus on the FASTGlobal surface removal and the migration guidance update.

📑 Test Plan

• Run the existing packages/fast-element/src/platform.pw.spec.ts Chromium Playwright spec.
• npm run build -w @microsoft/fast-element
• npm run prebuild -w sites/website
• npm run build
• npm run biome:check
• npm run checkchange -- --branch origin/releases/fast-element-v3

✅ Checklist

General

• I have included a change request file using $ npm run change
• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.