Update Babel dependencies to v8

[Copilot]

Closes N/A

Changelog

New

• N/A

Changed

• Updated direct Babel dependencies across the repository to Babel 8.
• Replaced deprecated Babel proposal plugin usage with Babel 8 transform plugins.
• Preserved existing package transform behavior with local Babel 8-compatible dev-expression and expression replacement plugins.

Removed

• Removed unused legacy Babel bridge/polyfill and v7-only Babel plugin dev dependencies from @primer/react.

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; dev tooling update only, no consumer-facing runtime/API change

Testing & Reviewing

• npm run build
• npm test -- --run
• npm run type-check
• npm run lint
• npm run lint:css
• npm run format:diff
• Parallel validation: Code Review and CodeQL passed

[changeset-bot]

⚠️ No Changeset found

Latest commit: da8d8d0

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[github-actions]

⚠️ Action required

👋 Hi, this pull request contains changes to the source code that github/github-ui depends on. If you are GitHub staff, test these changes with github/github-ui using the integration workflow. If this doesn't work, you can also use the original workflow here. Check the integration testing docs for step-by-step instructions. Or, apply the integration-tests: skipped manually label to skip these checks.

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