Add Ruby 3.0, 3.1, 3.2, and 3.3 to CI, and retire 2.6

[parkr]

Ruby is moving forward, and Pages must follow or risk security issues and poor developer experience.

[parkr]

@yoannchaudet It looks like the Dockerfile-based CI is failing due to dropping support for Ruby 2.x:

#8 ERROR: process "/bin/sh -c gem update --system" did not complete successfully: exit code: 1
------
 > [ 3/12] RUN gem update --system:
14.32 ERROR:  Error installing rubygems-update:
14.32 	There are no versions of rubygems-update (= 3.5.3) compatible with your Ruby & RubyGems
14.32 	rubygems-update requires Ruby version >= 3.0.0. The current ruby version is 2.7.3.183.
14.32 ERROR:  While executing gem ... (NoMethodError)
14.32     undefined method `version' for nil:NilClass
14.34 Updating rubygems-update
------
Dockerfile:11
--------------------
   9 |         nodejs
  10 |     
  11 | >>> RUN gem update --system
  12 |     
  13 |     COPY .git /src/gh/pages-gem/.git
--------------------

[yoannchaudet]

Hopefully #901 is equivalent 🤞.