Upgrades Node.js to v18

[willroberts]

Summary

Upgrades from Node.js v16 to v18 (the new LTS release), and switches from alpine images to slim images (slightly larger size, but Debian-based with official Node.js builds).

Build changes (docker/, gulp/, scripts/, etc.):

• Switches from node:16-alpine to node:18-slim images
• Bumps to v1.97.7 so we can build new backend images

Infrastructure changes (.github/, terraform/, etc.):

• Uses Node.js v18 in workflows
• Switches to -slim image in integration test workflow and adds bcrypt+git dependencies

Testing

Have you have tested your changes in the following scenarios? Feel free to check off scenarios which don't apply.

• [x] Starting backend services locally with docker compose up succeeds.
• [x] I am able to create a new user and log in locally.
• [x] I am able to complete a practice game locally.
• [x] I am able to complete a purchase of Orbs, etc.