Investigate `pnpm runtime set node <version> -g` to install Node.js

from @Samk13:

Reading through the [pnpm docs](https://pnpm.io/cli/runtime), it looks like we could potentially simplify this Dockerfile further by relying on `pnpm runtime set node -g` to [install Node.js](https://pnpm.io/docker#installing-nodejs) as well.

Since we're already standardizing on pnpm, that could let us drop the NodeSource setup and the `nodejs`/`npm` apt packages entirely, and have pnpm manage both the package manager and runtime versions.

Might be worth exploring in a follow-up if it aligns with our support and deployment expectations.

_Originally posted by @Samk13 in https://github.com/inveniosoftware/docker-invenio/issues/96#issuecomment-4753344515_

Start by reviewing the Dockerfile's current NodeSource setup and nodejs/npm apt packages. Read the linked pnpm runtime and Docker documentation, then check whether pnpm can manage the required Node.js versions within the project's support and deployment expectations. Done means a documented decision on whether this replacement is viable.

Written by the indexing model from the issue text.