Update .nvmrc to use node 14

[walbo]

Description

Latest LTS in now node 16 and ships with npm 8 which uses lock version 2. Using v2 is blocked until #36041 is merged.

Update .nvmrc to use node 14. This the previous LTS version (now a maintenance LTS) and used in all the CI test.

This PR should be reverted once #36041 is merged (or added to that PR).

How has this been tested?

Confirm nvm use in Gutenberg repo sets the proper node/npm versions.

Types of changes

Updates .nvmrc config

[gziolo]

It probably makes sense to keep the version explicit because every time there is s new active LTS version we always need to propagate changes to WordPress core and ensure that the deploy stack is aligned.

The other issue is that we always have to update this lts/* when we create branches for previous major WordPress releases.

@desrosj and @peterwilsoncc since you encounter some issues with it in the past, what do you think?

If we go this route we need to ensure that the documentation for contributors is updated so they are aware which version of Node.js is recommended. We could even send them to the updated file with the note that we aim for the active LTS version but in practice it can diverge. Example:
https://github.com/WordPress/gutenberg/blob/trunk/docs/contributors/code/getting-started-with-code-contribution.md#prerequisites

[gziolo]

We landed #36744 that contains the same changes together with all necessary changes to the documentation.