ESLint config format is deprecated

[MajorLift]

• The config format that we currently use is deprecated starting in ESLint v9: https://eslint.org/docs/latest/use/configure/configuration-files-deprecated
• The @metamask/eslint-config monorepo has migrated to the new flat config format, which makes it necessary for all downstream repos that use @metamask/-namespaced configs to perform this migration as well.
  • ESLint config format is deprecated eslint-config#362
  • BREAKING: Bump ESLint to ^9.11.1, bump related ESLint dependencies, and rewrite configs to use flat configs eslint-config#370
• Analogous PR in ts-bridge that can serve as a reference: Bump ESLint to 9.11.1 and rewrite configs to flat config ts-bridge/ts-bridge#63

[mcmire]

There are also some related PRs from the Ocap Kernel team we can use for inspiration:

• chore: Upgrade Eslint from v8 to v9 (flat config) ocap-kernel#139
• chore: Refactor ESLint scripts and configs ocap-kernel#176
• chore: Fix typechecks ocap-kernel#192