chore: move to eslint v9 (#7041)

- Upgrades eslint from v8 to v9
- Upgrades all other eslint packages. We will have to do a new
full-project lint, as new rules have been added
- Upgrades husky from v8 to v9
- Upgrades lint-staged from v14 to v15
- Moves the old .eslintrc.cjs file format to the new eslint.config.js
flat file format.

Previously, we were very specific regarding which rules are applied to
which files. Now that `extends` is no longer a thing, I have to use
deepMerge & imports instead.

This is rather uncommon and is not a documented pattern - e.g.
typescript-eslint docs want us to add the default typescript-eslint
rules to the top-level & then disable it in files using the
disable-typechecked config.

However, I hate this opt-out approach. The way I did it here adds a lot
of clarity as to which rules are applied to which files, and is pretty
easy to read. Much less black magic

## .eslintignore

These files are no longer supported (see
https://eslint.org/docs/latest/use/configure/migration-guide#ignoring-files).
I moved the entries to the ignores property in the eslint config. => one
less file in each package folder!

test/eslint.config.js

@@ -0,0 +1,80 @@
+import { rootEslintConfig, rootParserOptions } from '../eslint.config.js'
+import payloadPlugin from '@payloadcms/eslint-plugin'
+import playwright from 'eslint-plugin-playwright'
+
+/** @typedef {import('eslint').Linter.FlatConfig} */
+let FlatConfig
+
+/** @type {FlatConfig[]} */
+export const index = [
+  ...rootEslintConfig,
+  {
+    languageOptions: {
+      parserOptions: {
+        project: './tsconfig.json',
+        tsconfigDirName: import.meta.dirname,
+        ...rootParserOptions,
+      },
+    },
+    plugins: {
+      payload: payloadPlugin,
+    },
+    rules: {
+      'payload/no-relative-monorepo-imports': 'error',
+    },
+  },
+  {
+    files: ['**/*.ts'],
+    rules: {
+      'payload/no-jsx-import-statements': 'warn',
+      'payload/no-relative-monorepo-imports': 'error',
+      '@typescript-eslint/no-unsafe-assignment': 'off',
+      '@typescript-eslint/no-use-before-define': 'off',
+      // turn the @typescript-eslint/unbound-method rule off *only* for test files. See https://typescript-eslint.io/rules/unbound-method/#when-not-to-use-it
+      '@typescript-eslint/unbound-method': 'off',
+      'no-console': 'off',
+      'perfectionist/sort-objects': 'off',
+    },
+  },
+  {
+    files: ['**/*.int.spec.ts', '**/int.spec.ts'],
+    rules: {
+      '@typescript-eslint/no-explicit-any': 'off',
+      'jest/prefer-strict-equal': 'off',
+    },
+  },
+  {
+    ...playwright.configs['flat/recommended'],
+    files: ['**/*.e2e.spec.ts', '**/e2e.spec.ts', 'helpers.ts'],
+  },
+  {
+    files: ['**/*.e2e.spec.ts', '**/e2e.spec.ts', 'helpers.ts'],
+    rules: {
+      'payload/no-relative-monorepo-imports': 'error',
+      '@typescript-eslint/no-unsafe-assignment': 'off',
+      '@typescript-eslint/no-use-before-define': 'off',
+      'jest/consistent-test-it': 'off',
+      'jest/expect-expect': 'off',
+      'jest/no-test-callback': 'off',
+      'jest/prefer-strict-equal': 'off',
+      'jest/require-top-level-describe': 'off',
+      'jest-dom/prefer-to-have-attribute': 'off',
+      'playwright/prefer-web-first-assertions': 'error',
+      'payload/no-flaky-assertions': 'warn',
+      'payload/no-wait-function': 'warn',
+      // Enable the no-non-retryable-assertions rule ONLY for hunting for flakes
+      // 'payload/no-non-retryable-assertions': 'error',
+    },
+  },
+  {
+    files: ['*.e2e.ts'],
+    rules: {
+      'payload/no-relative-monorepo-imports': 'error',
+      '@typescript-eslint/no-unsafe-assignment': 'off',
+      '@typescript-eslint/no-use-before-define': 'off',
+      'jest/expect-expect': 'off',
+    },
+  },
+]
+
+export default index