Update Rails to 7.2 and next Rails to 8.0

Only needed some changes to the serializing in Conference.

.rubocop_todo.yml

@@ -1,6 +1,6 @@
 # This configuration was generated by
 # `rubocop --auto-gen-config`
-# on 2025-08-12 13:33:30 UTC using RuboCop version 1.79.2.
+# on 2025-08-12 15:04:30 UTC using RuboCop version 1.79.2.
 # The point is for the user to remove these configuration records
 # one by one as the offenses are removed from the code base.
 # Note that changes in the inspected code, or installation of new
@@ -545,7 +545,7 @@ Metrics/BlockNesting:
 # Offense count: 14
 # Configuration parameters: CountComments, CountAsOne.
 Metrics/ClassLength:
-  Max: 646
+  Max: 655
 
 # Offense count: 24
 # Configuration parameters: AllowedMethods, AllowedPatterns.
@@ -1810,6 +1810,7 @@ Style/NumericPredicate:
     - 'app/helpers/application_helper.rb'
     - 'app/helpers/date_time_helper.rb'
     - 'app/helpers/format_helper.rb'
+    - 'app/lib/rails_version.rb'
     - 'app/models/cfp.rb'
     - 'app/models/conference.rb'
     - 'app/models/event.rb'
@@ -2074,7 +2075,7 @@ Style/WordArray:
   EnforcedStyle: percent
   MinSize: 6
 
-# Offense count: 431
+# Offense count: 430
 # This cop supports safe autocorrection (--autocorrect).
 # Configuration parameters: AllowHeredoc, AllowURI, AllowQualifiedName, URISchemes, IgnoreCopDirectives, AllowedPatterns, SplitStrings.
 # URISchemes: http, https