[Future] Upgrade to Rails 7.2

[Splines]

Things to consider

• [ ] Check for Param equality check violations in production log, see this comment
• [ ] New cache format: https://github.com/MaMpf-HD/mampf/issues/554#issuecomment-2045979728 (hopefully already done when closing issue #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.