Pin boostrap version

the new version of bootstrap doesn't play nice with an old version of
postcss-values-parser we're stuck with because webpacker

this is significantly easier than migrating to shakapacker or
jsbundling-rails

see #315

config/initializers/check_env.rb

@@ -4,5 +4,5 @@
 
 if Rails.env.production? && Rails.root.join('example.env').read.include?(ENV.fetch('RAILS_SECRET_KEY_BASE'))
   raise 'RAILS_SECRET_KEY_BASE is unchanged from example.env. ' \
-    'Generate a new one with `ruby -rsecurerandom -e "puts SecureRandom.hex(64)"`'
+    'Generate a new one with `bundle exec rails secret`'
 end

variants/frontend-bootstrap/template.rb

@@ -1,6 +1,6 @@
 source_paths.unshift(File.dirname(__FILE__))
 
-yarn_add_dependencies %w[bootstrap @popperjs/core]
+yarn_add_dependencies %w[bootstrap@5.1.3 @popperjs/core]
 run "yarn install"
 
 copy_file "app/frontend/js/bootstrap.js", force: true