Update Jekyll and the building process

[napoly]

Upgrading Jekyll to 3.9.3 and Ubuntu to latest (22.04). Had to drop Ruby 2.6 support as running it backward compatible would require downgrading some of the dependencies. I assume the server is running already ruby 2.7 and higher.

[woodser]

Maybe it looks ok to @erciccione?

[erciccione]

Looks ok. Specifying webrick as a dependency shouldn't be necessary anymore.

[napoly]

Seems it has to be there for Ruby 3.x and higher as written in docs:

If you are using Ruby version 3.0.0 or higher, step 5 may fail. You may fix it by adding webrick to your dependencies: bundle add webrick

.. and fails on my localhost:
/var/lib/gems/3.0.0/gems/jekyll-3.9.3/lib/jekyll/commands/serve/servlet.rb:3:in `require': cannot load such file -- webrick (LoadError)

But there is another major version of Jekyll 4.x and they have added this dependency directly there at version 4.3.0. But upgrading directly to 4.3.2 version as of today would require to drop https://rubygems.org/gems/github-pages as the latest version 228 requires Jekyll 3.9.3.

So this PR is more about passing the build. Updating pipelines. Than having the newest Jekyll up and running.