feature: upgrade to Astro v5 & Node.js v22

[jbmoelker]

Changes

• Upgrade to Astro v5 (stable) and Node.js v22 (latest LTS version).
• Upgrade related packages (adapters, linter, etc).

Associated issue

Resolves #188

How to test

1. Open preview link
2. Verify all pages still work
3. Verify preview mode still works

Checklist

• I have performed a self-review of my own code
• I have made sure that my PR is easy to review (not too big, includes comments)
• I have made updated relevant documentation files (in project README, docs/, etc)
• I have added a decision log entry if the change affects the architecture or changes a significant technology
• I have notified a reviewer

[cloudflare-workers-and-pages]

Deploying head-start with    Cloudflare Pages

Latest commit:	e2f87ba
Status:	✅  Deploy successful!
Preview URL:	https://6c1dd9bf.head-start.pages.dev
Branch Preview URL:	https://feat-astro-v5-nodejs-v22.head-start.pages.dev

View logs

[jbmoelker]

Blocked as Astro v5 uses Vite v6, while Vitest still uses (/ has reverted to) Vite v5 🤷. Issue has been filled: withastro/astro#12662 .
We could try suggested override as a workaround or wait for Vitest to update to Vite v6.