Deprecate update_set_schema() for removal

Problem/Motivation

Spotted while reviewing #2124069: Convert schema.inc to the update.update_hook_registry service (UpdateHookRegistry)

update_set_schema() is not used in core except in a test:

core/modules/system/tests/src/Functional/UpdateSystem/UpdateSchemaTest.php
82:    // Test the update_set_schema() utility function.
84:    update_set_schema('update_test_schema', 8003);

core/includes/update.inc
157:function update_set_schema($module, $schema_version) {

It was introduced for a single use case when we had a Drupal 7 to 8 upgrade path prior to requiring migrations: #2001310: Disallow firing hooks during update

The function comment notes that "This function is rarely necessary."

It is unused in contrib: http://grep.xnddx.ru/search?text=update_set_schema&filename=

Steps to reproduce

Proposed resolution

Deprecate it for removal in Drupal 10 with no replacement.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet