chore: set node version to 12.18.4

What does this MR do?

Changes required Node version to align with that of the GDK: https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/cd6c178f91559cd31ef08066e17314ce27d9a0ff/.tool-versions#L11

Does this MR meet the acceptance criteria?

Conformity

• [-] Code review guidelines.
• GitLab UI's contributing guidlines.
• [-] If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• [-] If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• [-] If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • CustomersDot: mr_url
  • Status Page: mr_url
• [-] Added the ~"component:*" label(s) if applicable.