Upgrade to ESLint v9

[jtfairbank]

It looks like there's a new major version of ESLint out, and we should upgrade it. The config file format changed, so this will require a manual update (renovate can't do it automatically).

Please take into consideration:

• The basic ESLint update - see https://eslint.org/docs/latest/use/configure/migration-guide
• Typescript Linting - see https://typescript-eslint.io/
• NextJS linting - see https://nextjs.org/blog/next-15#eslint-9-support

[jtfairbank]

Hey @IAmJSD! Just checking up on this one- things going ok with it?