Summary:
Upstream PG 3c6f8c011f85df7b35c32f4ccaac5c86c9064a4a removes
pg_strtouint64 and replaces usages with strtoull or strtou64.  YB PG
15.2 initial merge 55782d561e55ef972f2470a4ae887dd791bb4a97 preserves
this pg_strtouint64, saying it is still used.  That seems short-sighted.

Remove pg_strtouint64 for good and replace usages appropriately.

Jira: DB-16286

Test Plan:
jenkins

Close: #26870

Reviewers: kfranz

Reviewed By: kfranz

Subscribers: kfranz, yql

Differential Revision: https://phorge.dev.yugabyte.com/D43292