chore(deps): update eslint and related deps to v9

[joshblack]

Closes https://github.com/github/primer/issues/4155

This is take 2 for updating the project to use ESLint v9. The previous attempt ended up having a larger than expected diff so I updated some plugins and then opened this up again so that it was easier to merge.

Changelog

New

Changed

• Update eslint to v9
• Migrate our eslint config to the new flat config
• Address new eslint errors, mostly from removed/replaced rules

Removed

[changeset-bot]

⚠️ No Changeset found

Latest commit: 2d5c981

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[github-actions]

👋 Hi, this pull request contains changes to the source code that github/github depends on. If you are GitHub staff, we recommend testing these changes with github/github using the integration workflow. Thanks!

[Copilot]

Pull Request Overview

This PR upgrades the repository to ESLint v9 by migrating to the new flat config format, updating related dependencies, and adjusting code to satisfy new lint rules and plugin changes.

• Migrated from legacy .eslintrc.js to eslint.config.mjs with flat config and updated plugin rules
• Bumped ESLint/core plugins to v9+ in package.json
• Replaced deprecated event handlers (onClick → onSelect) and added temporary disables for unused maps

Reviewed Changes

Copilot reviewed 37 out of 37 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/react/src/Overlay/Overlay.tsx	Disabled unused widthMap for new lint setup
packages/react/src/NavList/NavList.tsx	Swapped onClick → onSelect on navigation items
packages/react/src/Dialog/Dialog.tsx	Disabled unused heightMap/widthMap for lint
packages/react/src/BranchName/tests/BranchName.types.test.tsx	Removed redundant ESLint disables in type tests
packages/react/src/BranchName/BranchName.tsx	Updated ESLint disable from ban-types to new rule
packages/react/src/ActionMenu/ActionMenu.examples.stories.tsx	Replaced onClick → onSelect in example stories
packages/react/src/ActionList/Item.tsx	Updated ESLint disable to no-unsafe-function-type
packages/react/src/ActionList/ActionListContainerContext.tsx	Updated ESLint disable to no-unsafe-function-type
packages/react/src/ActionList/ActionList.features.stories.tsx	Swapped onClick → onSelect in feature stories
packages/react/src/ActionList/ActionList.dev.stories.tsx	Swapped onClick → onSelect in dev stories
packages/react/src/ActionBar/ActionBar.tsx	Swapped onClick → onSelect in ActionBar items
packages/react/src/ActionBar/ActionBar.examples.stories.tsx	Swapped onClick → onSelect in examples
packages/react/.storybook/theme.js	Added ESLint disable for import/named in Storybook
packages/react/.storybook/src/accessibility-tool.tsx	Added disables and deps for SSR-friendly lint rules
packages/postcss-preset-primer/src/index.js	Switched @ts-ignore to @ts-expect-error
package.json	Bumped ESLint and related packages to v9+
eslint.config.mjs	Introduced new flat ESLint configuration
e2e/components/ActionList.test.ts	Commented-out legacy tests removed for clarity
.eslintrc.js	Removed legacy ESLint config
.eslintignore	Removed redundant ignore file

Comments suppressed due to low confidence (3)

e2e/components/ActionList.test.ts:206

• Instead of commenting out tests, use test.skip or update the feature-flag logic so the test remains active and coverage is preserved.

// removing this temporarily as there is a slight diff betqeen default and enabled CSS feature flag that feels like a non-issue

packages/react/src/Overlay/Overlay.tsx:40

• Consider removing the unused widthMap constant entirely, or exporting it if it will be used soon, to avoid disabling the unused-vars rule.

// eslint-disable-next-line @typescript-eslint/no-unused-vars

packages/react/src/Dialog/Dialog.tsx:171

• The heightMap and widthMap objects are not referenced. Remove them or integrate their usage to clean up the code.

// eslint-disable-next-line @typescript-eslint/no-unused-vars

[github-actions]

size-limit report 📦

Path	Size
packages/react/dist/browser.esm.js	94.4 KB (+0.05% 🔺)
packages/react/dist/browser.umd.js	94.45 KB (-0.12% 🔽)

[francinelucca]

@copilot can you fix the broken CI here?