Upgrade Nuxt to v2.11.0

This upgrades Nuxt to v2.11.0. See the full changelog here: https://github.com/nuxt/nuxt.js/releases

Since Nuxt depends on vue@2.6.11, vue-template-compiler had to be updated to v2.6.11 as well, and a resolutions field has been added to lock vue to v2.6.11. This is all done to prevent packages versions mismatches.

This Nuxt upgrade is needed to unblock a GitLab UI upgrade: !1699 (merged) A recent change in GitLab UI introduced an optional chaining operator, which isn't supported in our current version of Nuxt. By bumping Nuxt to more recent version, we also get an updated Babel config, which does support optional chaining.