See https://gitlab.com/groups/gitlab-org/-/epics/9477. node@18 is the
latest LTS and the new version has some nice features we might end up
using (e.g. more parity on the Intl API with browsers).

In order to ensure that node version bump result in re-compiling of
assets, we also tell the asset hashing that any change in .nvmrc
requires a full recompilation.

Changelog: changed

This is consistent with gitlab-ui and gitlab-docs

This matches the version required by GDK's `.tool-versions` file

to match what is required by GDK

This updates our build images to use node@12. We are also adding a
sentence to the from-source installation that while we require a minimum
version of node@8, we also recommend node@12. Last but not least, we
change the version of node in the .nvmrc so that developers using nvm
can automatically use node@12 with the project