chore: remove legacy globalThis polyfill

[janechu]

Pull Request

📖 Description

• Remove the legacy globalThis polyfill from @microsoft/fast-element while
  keeping the requestIdleCallback / cancelIdleCallback fallback.
• Document the new v3 runtime expectation in README.md, DESIGN.md,
  MIGRATION.md, and the 3.x browser support page.
• Regenerate the fast-element size docs to capture the smaller bundle output.

🎫 Issues

• Closes Remove globalThis #7289

👩‍💻 Reviewer Notes

• The only runtime code change is in
  packages/fast-element/src/polyfills.ts; the remaining edits are
  documentation, migration guidance, the change file, and generated size docs.
• npm run test still hits an existing @microsoft/fast-html
nested-elements failure on this branch, and
  npm run test -w @microsoft/fast-element still sees existing Playwright
  connection-refused / flaky failures unrelated to this polyfill removal.

📑 Test Plan

• npm run build
• npm run biome:check
• npm run checkchange
• node --input-type=module -e "delete globalThis.requestIdleCallback; delete globalThis.cancelIdleCallback; await import('./packages/fast-element/dist/esm/polyfills.js'); console.log(typeof globalThis.requestIdleCallback, typeof globalThis.cancelIdleCallback);"
• npm run test (currently fails in pre-existing packages/fast-html/test/fixtures/scenarios/nested-elements/nested-elements.spec.ts)
• npm run test -w @microsoft/fast-element (currently fails in existing Playwright instability / ERR_CONNECTION_REFUSED cases on this branch)

✅ 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.