Notice the upgraded version is 1.5 and the new version is 1.8
I would think somewhere in the upgrade of the schema there should have been a create extension pg_stat_statements ?
That would be a faulty assumption. Modules do not get upgraded during a server version upgrade. This is a good thing, IMO.
This is from the documentation of pg_upgrade
Install any custom shared object files (or DLLs) used by the old cluster into the new cluster, e.g., pgcrypto.so, whether they are from contrib or some other source. Do not install the schema definitions, e.g., CREATE EXTENSION pgcrypto, because these will be upgraded from the old cluster. Also, any custom full text search files (dictionary, synonym, thesaurus, stop words) must also be copied to the new cluster.
If indeed modules do not get upgraded then the above is confusing at best, and misleading at worst.
