Update Jekyll to latest 3.x to support Ruby 3

Debian ships Ruby 3.1.2 and Jekyll 3.9.5 manages to build and serve the
pages.

I had to explicitly add `kramdown-parser-gfm`. Ruby 3 moved `rexml` and
`webrick` out of the standard library.

Also update the Dockerfile base to Ruby 3.3.

Change-Id: I8702537dde1acd5ce272e1babac70ec480781353
3 files changed