Update to supported version of Node.js in the workflows and on the server

[ryonakano]

Problem

The following two workflows uses Node.js 18 which was EOL.

• https://github.com/elementary/website/blob/f5758d8337cdcdc08107aba56ac614a09d785c76/.github/workflows/ci.yml
• https://github.com/elementary/website/blob/f5758d8337cdcdc08107aba56ac614a09d785c76/.github/workflows/update_translations.yml

like this:

      - name: Setup Node.js
        uses: actions/setup-node@v6.3.0
        with:
          node-version: 18

FYI supported list of Node.js: https://nodejs.org/en/about/previous-releases

Proposal

These workflow files should use supported version of Node.js.

Prior Art (Optional)

No response

[lewisgoddard]

We are currently running Node 18 on server, so this should probably become a wider "rebuild web-hosting droplet" task.

$ node -v
v18.17.1
$ npm -v
9.6.7