Upgrade Hugo to 0.120.3, and Node engine constraint

[chalin]

• Contributes to Release 0.7.2 preparation #1666
• Upgrades Hugo to 0.120.3
• Updates min Node version to be currently active Node LTS version (20.x)

The only change in generated files (other than the Hugo version) is the Shortcodes page:

$ (cd public && git diff -bw --ignore-blank-lines -I "Hugo 0.1" -- . ':(exclude)*.xml') | grep ^diff | grep -v print
diff --git a/docs/adding-content/shortcodes/index.html b/docs/adding-content/shortcodes/index.html

And the only difference is in syntax highlighting of Java:

Before

After