rouge update is blocked by
 https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=971841

--- a/Gemfile
+++ b/Gemfile
@@ -155,7 +155,7 @@
 gem 'asciidoctor-include-ext', '~> 0.3.1', require: false
 gem 'asciidoctor-plantuml', '~> 0.0.12'
 gem 'asciidoctor-kroki', '~> 0.2.2', require: false
-gem 'rouge', '~> 3.26'
+gem 'rouge', '~> 3.21'
 gem 'truncato', '~> 0.7.11'
 gem 'bootstrap_form', '~> 4.2'
 gem 'nokogiri', '~> 1.10', '>= 1.10.9'