Bump .nvmrc to 16

[Daeraxa]

Bumping to default to 16 (not specified an exact version, can change to lts/* if people prefer or a specific 16.x.x version).

The current version in here doesn't even work (node 12) so we lose nothing by bumping to at least 14 and stops install failing when using deeper nvm integration.

[mauricioszabo]

There's little difference in bumping versions because most packages are compiled via electron-rebuilder, but I see no problems on bumping this :)

[Daeraxa]

Its mostly just helpful for those using deeper nvm integration.
If I change to a directory with the older .nvmrc then it automatically uses node 12 which then fails on yarn install. This way you can just nvm use in the root of the project and it will always go to whatever latest node 16 you have (or tell you to install it).

Also potentially means I can update the build docs to say just run nvm install instead of nvm install 16 so it just automatically tracks whatever the recommended version of node is for the project (so long as this file is kept updated).

[confused-Techie]

This looks good to me. And love getting our docs simpler for everyone.

With an additional approval, I'll approve here and merge.