Deprecate the hierarchy flag of Config Setup

Summary

Deprecate the hierarchical flag of Quantum\Config\Setup in 3.1.2, with no change in behavior. It is removed in 3.2.0.

The flag controls one thing: when a config file is not found in the module's folder, Config::resolveFilePath() falls back to shared/<prefix>/<file>.php. That is a fallback or override rule, not a hierarchy. The name suggests the "hierarchical" in HMVC, which means nested sub-requests between MVC triads and has nothing to do with config lookup.
The flag defaults to true, and nothing in src turns it off: every new Setup('config', '<name>') call uses the default. Only SetupTest calls getHierarchy() and setHierarchy().
Turning it off is a trap. A global config such as new Setup('config', 'app') has no module, so its first path is <base>/config/app.php, which does not exist in a project. The files live in shared/config, and only the fallback finds them.

Mark Setup::setHierarchy(), Setup::getHierarchy() and the $hierarchical constructor parameter @deprecated, saying that the shared fallback is always on from 3.2.0.
Raise E_USER_DEPRECATED when the setter is called or when the constructor receives the third argument (func_num_args() > 2), with a message that names the removal in 3.2.0. The error handler already logs deprecations as warnings, so nothing stops. The default true does not raise anything.
Behavior stays exactly as it is.
Describe the lookup in the docs as the "shared fallback" or "lookup order", not as a "hierarchy" (advanced-features/configuration.md). That is a change in the docs repository.

the two methods and the constructor parameter are marked @deprecated
calling setHierarchy() or passing the third constructor argument raises E_USER_DEPRECATED, and the default call does not; a test covers both
the lookup result is unchanged for true and false, and the existing tests still pass
composer test, composer cs:check, composer phpstan and composer rector:check pass on every PHP version in the CI matrix
the changelog lists the deprecation