enforce rouge gem to old version to fix highlight issue

[crazy-max]

fixes #14788

before we were using rouge 2.2.1 and looks like migration to rouge 3.28.0 upgraded by Jekyll 4 introduces a regression with syntax highlighting.

we can't just enforce rouge 2.2.1:

#13 3.929 Bundler could not find compatible versions for gem "rouge":
#13 3.929   In Gemfile:
#13 3.929     rouge (= 2.2.1)
#13 3.929
#13 3.929     jekyll was resolved to 4.2.2, which depends on
#13 3.929       rouge (~> 3.0)

so temporarily downgrade to Jekyll 3.8.6 and also pin rouge to 2.2.1.

Signed-off-by: CrazyMax crazy-max@users.noreply.github.com

[thaJeztah]

LGTM (waiting for the preview to be created)

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[thaJeztah]

Looking good