Failures in `Mathlib.Computability.Primrec` after Std bump

Stream: mathlib4

[Scott Morrison]

#9039 is the lastest bump to Mathlib's Std dependency. It has some failures that I'd like some help with, in Mathlib.Computability.Primrec. The definition of List.indexOf has changed (the == test has been flipped), and thus the Primrec results talking about List.indexOf need updating too.

@Mario Carneiro (listed as author) or @Praneeth Kolichala (who ported the file) might be able to help out?

[Mario Carneiro]

fixed

[Scott Morrison]

This PR is ready for review. We have lots more Std bumps coming, so it would be great to get this in.