Update to node@12

What does this MR do?

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

See https://gitlab.com/gitlab-org/gitlab-ce/issues/63205

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team