Support non-public schema for PostgreSQL 15+

### Description

For PostgreSQL setup, the doc says:

-- PostgreSQL 15 requires additional privileges:
GRANT ALL ON SCHEMA public TO airflow_user;

For security-concerned customers that do not want to use the public schema, it would be nice if the schema to use could be configurable.

### Use case/motivation

For security sake, use a schema other than public.

### Related issues

_No response_

### Are you willing to submit a PR?

- [X] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

The issue identifies PostgreSQL setup and the public schema as the scope, but names no files, tests, or entry points. Begin by locating where Airflow configures PostgreSQL schemas and determine how a non-public schema should be selected. Done means security-conscious customers can configure and use a schema other than public.

Written by the indexing model from the issue text.