ActiveModel compatibility between Rails 7 and 8

[philippthun]

The internal representation of the validation context has changed between Rails 7 and 8. As we don't use validation contexts, it only has to be ensured that a newly introduced class exists.

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CATS has been executed with the following Rails versions:

I.   api with Rails 7 + worker with Rails 8
       -> old job serialization format + new deserialization code
       => No errors

II.  api with Rails 8 + worker with Rails 7
       -> new job serialization format + old deserialization code
       => Errors
         Deserialization for job '<guid>' failed, rescheduling it...
         ... FAILED permanently with Delayed::DeserializationError: Job failed to load: undefined class/module ActiveModel::ValidationContext.

III. api with Rails 8 + worker with Rails 7 + this change
       -> new job serialization format + old deserialization code with compatibility patch
       => No errors

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