chore(fast-ssr): deprecate package in favor of @microsoft/fast-build and @microsoft/webui

[janechu]

Pull Request

📖 Description

Marks @microsoft/fast-ssr as deprecated in favor of @microsoft/fast-build and @microsoft/webui, which provide the recommended path forward for server-side rendering of FAST Web Components.

• Adds a "deprecated" field to package.json so npm displays a deprecation warning when the package is installed.
• Updates README.md with a prominent deprecation notice linking to the successor packages.

📑 Test Plan

Documentation and metadata-only change. All existing tests continue to pass unchanged.

✅ 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.

⏭ Next Steps

• Run npm deprecate @microsoft/fast-ssr "This package is deprecated. Please use @microsoft/fast-build and @microsoft/webui instead." on publish to mark the package as deprecated on the npm registry.
• Delete the package from the repository