[CI] Update Hugo to 0.139.4

[chalin]

Updates Hugo to 0.139.4. The main change that has an impact is:

• tpl/tplimpl: Escape Markdown attributes in render hooks and shortcodes gohugoio/hugo@54398f8 @jmooring

The generated site-file diff:

$ (cd public && git diff -bw -I 'img src|img alt|iframe|Hugo 0' -- . ':(exclude)*.xml') | grep ^diff
diff --git a/blog/2023/euwg-future-of-observability-panel/index.html b/blog/2023/euwg-future-of-observability-panel/index.html
diff --git a/blog/2023/humans-of-otel/index.html b/blog/2023/humans-of-otel/index.html
diff --git a/blog/2024/humans-of-otel-eu-2024/index.html b/blog/2024/humans-of-otel-eu-2024/index.html
diff --git a/site/index.html b/site/index.html

The blog pages contain iframe element cleanup (I can't get the -I expression to ignore those), and the main index is listed because of the timestamp.