Bump node.js version to 18

[rmunn]

Fixes #1792

Description

Node 22 just came out, so we want to upgrade off of Node 16 which is going end-of-life today (end of April 2024). As a first step, we'll move to Node 18/.

This requires upgrading webpack, as webpack 5.27.1 used a feature of older Node versions that is deprecated in Node 18 and above. No other NPM package upgrades have been required yet; once we're running a modern version of Node, I'll upgrade more NPM packages.

Screenshots

N/A

Checklist

• I have labeled my PR with: bug, feature, engineering, security fix or testing
• I have performed a self-review of my own code
• I have reviewed the title & description of this PR which I will use as the squashed PR commit message
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• I have enabled auto-merge (optional)

Testing

• Run make, make sure site still loads. That's about it.

[github-actions]

Unit Test Results

362 tests   362 ✅  12s ⏱️
 37 suites    0 💤
  1 files      0 ❌

Results for commit 484b3a5.

[megahirt]

Actually, Node 16 went EOL in Sept 2023.

[megahirt]

Superseded by #1797