Build: Update ruby from 2.7 to 3.0

* Ubuntu 20.04 comes with Ruby 2.7, same as we were installing.
* Ubuntu 22.04 comes with Ruby 3.0, which is already supported
  by Jekyll 4.x, as is Ruby 3.1 and Ruby 3.2.

The following is written at actions/runner-images#2193:
> Officially, Ruby 2.4 and 2.5 don't have pre-installed Bundler.
> Bundler was built-in only in Ruby 2.6 and 2.7.
> Our previous binaries were customized and contained bundler for Ruby 2.4, 2.5.
> […]
> We are switching Ruby binaries to official ones provided by Ruby.

In complete contradiction to this, Bundler is NOT preinstalled on
ubuntu-20.04 as of writing, thus still requiring the use of the
setup-ruby action (or installing bundler manually).

Author: Krinkle

.github/workflows/github-pages.yaml

@@ -23,7 +23,7 @@ jobs:
 
       - uses: ruby/setup-ruby@v1
         with:
-          ruby-version: 2.7
+          ruby-version: 3.0
           bundler-cache: true
           # If your site is in a subdirectory
           # working-directory: ./