Assign privileges to a postgres user

Question

I am working on creating a postgres user and assign certain privileges to the user. These are privileges to execute certain functions with the end goal of being able to replicate data or change-data-capture.

Specifically, I am looking at the system privilege column of the postgres row in the table - https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/UsingWithRDS.MasterAccounts.html

I am running into issues granting execute privileges here. For example - grant execute on routine pg_terminate_backend() to <db_user>;

This returns an error - function pg_terminate_backend() does not exist However, I am able to query it in information_schema.routines table.

What would be the right to grant the privileges to a db user. I tried creation of a role and grant privileges but it shows the same error.