[Future] Upgrade to Rails 7.2

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Things to consider

• Check for Param equality check violations in production log, see this comment
• New cache format: Migrate to Rails 7.1 #554 (comment)
  (hopefully already done when closing issue Migrate to Rails 7.1 #554, see also this comment). For an explanation, see this comment
• coder keyword as seen here. One such occurrence (among others):

Please pass the coder as a keyword argument:

   serialize :quiz_graph, coder: QuizGraph
   (called from <class:Medium> at /usr/src/app/app/models/medium.rb:64)
   DEPRECATION WARNING: Passing the coder as positional argument is deprecated and will be removed in Rails 7.2.

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For the sunspot-related deprecation warning

DEPRECATION WARNING: BOLD is deprecated! Use MODES[:bold] instead.

see this issue. Unfortunately, it seems like sunspot only gets updates very sparingly.