Update Hugo to v0.150.1

What does this MR do and why?

• Updates Hugo to the latest release, v0.150.1
• Re-enables English fallbacks for untranslated JP pages. We had to disable this feature due to an issue in Hugo 0.146.0, which was resolved via https://github.com/gohugoio/hugo/pull/13995.
  • More info here: https://discourse.gohugo.io/t/path-warnings-for-multilingual-content-fallbacks-after-v0-146-0/55931/

Closes #500 (closed)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment.
2. Check out this branch.
3. Run make setup to install the new Hugo version.
4. Run make view and browse around the site. No changes expected.
5. Run a full build and check for errors: hugo --gc --minify --printPathWarnings --panicOnWarning
6. Enable Japanese in hugo.yaml, then check a full build again: hugo --gc --minify --printPathWarnings --panicOnWarning

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.