fix(hugo): replace config keys and template vars deprecated in Hugo v0.153–v0.158

[haarchri]

Fixes four deprecation warnings emitted by recent Hugo versions:

• languageCode → locale in config.yaml (deprecated in Hugo v0.158.0)
• module.mounts.includeFiles → files for the three content mounts in config.yaml (deprecated in Hugo v0.153.0), same glob patterns
• .Site.Data → hugo.Data in the geekboot theme shortcodes and partials (deprecated in Hugo v0.156.0)
• site.Language.LanguageCode → site.Language.Locale in the RSS template (deprecated in Hugo v0.158.0)
• bump hugo
• update security config for postcss config
• need to use node22

No content or rendering changes. Verified with a full hugo build at --logLevel warn, which now completes with zero warnings.

Fixes: #1137

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