[engsys] upgrade Eslint to v9

[jeremymeng]

One of the notable breaking changes is the moving to Flat Config and dropping support for legacy config files. To upgrade to v9, we need to

• update our eslint-plugin-azure-sdk to support Flat Config (eslint.config.{js,mjs})
  • PR [eslint-plugin] support Flat Config #29751
• migrate existing packages to use Flag Config
  • The changes need look easy enough to utilize automated transformation. Could use dev-tool admin list packages to apply the transformation for all packages in one go.
  • remove --ext command line option
• modify codegen to adapt to changes
  • removing of --ext option for example

Issues

• some plugins we are using today doesn't fully support ESLint v9 yet. They were enabled with the help of @eslint/compat

  • eslint-plugin-import Support eslint v9 import-js/eslint-plugin-import#2948
  • eslint-plugin-promise Support ESLint 9 and Flat config eslint-community/eslint-plugin-promise#449
  • eslint-plugin-tsdoc ESLint flat configurations? microsoft/tsdoc#374
  • eslint-plugin-no-only-tests ESLint flat configurations? levibuzolic/eslint-plugin-no-only-tests#43

• VSCode eslint plugin's support for v9 hasn't landed yet Enable Flat Config by default microsoft/vscode-eslint#1644

  • there's pre-release version that support it. Enable Flat Config by default microsoft/vscode-eslint#1644 (comment)