Merge #826: Bump Ruby to 2.7.4, update github-pages and html-proofer gems

bda840a Bump Ruby gems (Sjors Provoost)
790a9c0 Bump Ruby to 2.7.4 (Sjors Provoost)

Pull request description:

  This requires updating the server to Debian Stable Bullseye, cc TheBlueMatt.

  I tested a few pages locally and didn't notice any difference.

  I also ran the `htmlproofer` incantation from the README, which I updated to match `Makefile`. Only one minor change was needed to satisfy the updated version.

  Closes #825

ACKs for top commit:
  pinheadmz:
    ACK bda840a

Tree-SHA512: 18564dc939cdfebea1116fdd217b44f083c5e03f14605e30a937817e7c2460c3e4c4b67313ced87695bfad89b258d588a20289026f31d41537037727666b4dd4

Author: fanquake

.cirrus.yml

@@ -1,11 +1,11 @@
 container:
-  image: ruby:2.5.5
+  image: ruby:2.7.4
 
 task:
   name: website
   env:
     JEKYLL_ENV: production
-    NOKOGIRI_USE_SYSTEM_LIBRARIES: true # speeds up installation of html-proofer 
+    NOKOGIRI_USE_SYSTEM_LIBRARIES: true # speeds up installation of html-proofer
     RUBYOPT: "-KU -E utf-8:utf-8"
 
   bootstrap_script: gem install bundler

.ruby-version

@@ -1 +1 @@
-2.5.5
+2.7.4

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 '2.5.5'
+ruby '2.7.4'
 
 ## If you add a new Gem below, run `bundle install` to install it.
 group :development do