Update Rails to 7.2 and next Rails to 8.0

Only needed some changes to the serializing in Conference.

Gemfile

@@ -10,9 +10,9 @@ ruby '~> 3.3'
 
 # as web framework
 if next?
-  gem 'rails', '~> 7.1'
+  gem 'rails', '~> 8.0'
 else
-  gem 'rails', '~> 7.0'
+  gem 'rails', '~> 7.2'
 end
 
 # Use Puma as the app server
@@ -192,10 +192,6 @@ gem 'icalendar'
 # to tame logs
 gem 'lograge'
 
-# FIXME: https://github.com/ruby-concurrency/concurrent-ruby/commit/d7ce956dacd0b772273d39b8ed31a30cff7ecf38
-#        remove after updating to rails to >7.1
-gem 'concurrent-ruby', '1.3.4'
-
 group :development do
   # for static code analisys
   gem 'rubocop', require: false