Upgrade to Next.js 12, EUI 41.3

[pugnascotia]

This PR updates all dependencies apart from EUI. Notably, that means an upgrade of Next.js from v11 to v12. There were surprisingly few changes to make this work. ESLint had to be kept back because Next.js complained about using the latest version. I expect that to be relaxed in the future.

Future PRs:

• Upgrade EUI Actually, upgrading EUI turned out to be trivial so I've included it.
• Try using Next's build-in ESLint support in order to reduce config in the starter

[thompsongl]

Code changes LGTM. Spot checked the app locally and it all looks good except this console error after yarn dev:

[chandlerprall]

Thanks again Rory! Changes LGTM, tested with both dev & prod modes.

I pushed a complete regeneration of yarn.lock to opt into some additional updates & clean up duplicated packages.

[chandlerprall]

Code changes LGTM. Spot checked the app locally and it all looks good except this console error after yarn dev:

This comes from EuiSideNav and is an unconfigurable thing, https://github.com/elastic/eui/blob/main/src/components/side_nav/side_nav.tsx#L188

Thankfully, apart from this warning, nothing breaks (unless you're on a really old react@15 version) and React updates the id + aria-controls appropriately. I've opened elastic/eui#5418 to track this, and an SSR meta issue in EUI as well elastic/eui#5419