ruby: Update to 3.1.2 and debian bookworm

The build server uses debian bookworm which has ruby 3.1.2.

Author: achow101

.cirrus.yml

@@ -1,5 +1,5 @@
 container:
-  image: ruby:2.7.4
+  image: ruby:3.1.2
 
 env:
     JEKYLL_ENV: production
@@ -22,7 +22,7 @@ task:
 task:
   name: debian only
   container:
-    image: debian:bullseye
+    image: debian:bookworm
   install_script: 
     - apt update
     - >

.ruby-version

@@ -1 +1 @@
-2.7.4
+3.1.2

Gemfile

@@ -4,7 +4,7 @@ source 'https://rubygems.org'
 ## If you update the version here, also update it in .ruby-version, .cirrus.yml
 ## and README.md. Then push your branch and make sure Cirrus supports that
 ## version.
-ruby '2.7.4'
+ruby '3.1.2'
 
 ## If you add a new Gem below, run `bundle install` to install it.
 group :development do

README.md

@@ -41,7 +41,7 @@ permalink: /en/2016/01/01/short-title
 ## Building
 
 This website is based on [Jekyll](https://jekyllrb.com/).  To build
-locally, [install Ruby 2.7.4](https://gorails.com/setup) using system
+locally, [install Ruby 3.1.2](https://gorails.com/setup) using system
 packages, [rvm](https://rvm.io), [rbenv](https://github.com/rbenv/rbenv), or another method.
 Then clone this repository and change directory into it: