Upgrade Jekyll build to latest Ruby 3.2.2 from 2.7.8 (keep fontcustom on 2.7.8)

[TWiStErRob]

Jekyll 3.9.3 runs on Ruby 2.7.8 or 3.2.2 too. Fontcustom 2.0.0 only runs on Ruby 2.7.8, because of the following errors:

• Ruby 3.2 and 3.3: https://github.com/FontCustom/fontcustom/issues/399
• Ruby 3.0 and 3.1: https://github.com/FontCustom/fontcustom/issues/390

Content changes: ' → ' in generated XML files.

The rest of the conversation are deprecations, which I'll leave for now, as I want to see what's the state after Jekyll 4.