DrupalKernel::findSitePath() incorrectly checks for site directory without settings.php requirement

Problem/Motivation

Following up on Try to use is_file() instead of file_exists() in DrupalKernel::findSitePath():

The check for finding a site path in a multisite construction, without the requirement for the existence of settings.php, is broken since #1333940 has been merged.

Change on Gitlab: https://git.drupalcode.org/project/drupal/-/commit/b310d19833c0de5dcd487...

Where file_exists($directory) returns TRUE when a directory exists, is_file($directory) does not. It returns FALSE when the given path is not a file.

Returns true if the filename exists and is a regular file, false otherwise.
- https://www.php.net/manual/en/function.is-file.php

Steps to reproduce

1. Create the directory web/sites/example.org
2. Add $sites['example.org'] = 'example.org'; to web/sites/sites.php
3. To test the site path output you can use Drush: drush --uri=https://example.org php:eval 'echo \Drush\Drush::bootstrapManager()->bootstrap()->confPath(FALSE);'
4. Instead of the expected output sites/example.org, you'll get sites/default

Proposed resolution

The check (!$require_settings && is_file($app_root . '/sites/' . $site_id)) needs to be replaced with (!$require_settings && is_dir($app_root . '/sites/' . $site_id)).

Remaining tasks

1. Writing a patch
2. Testing the patch
3. Update core/tests/Drupal/Tests/Core/DrupalKernel/DrupalKernelTest.php to test changes
4. Merging the patch

User interface changes

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Introduced terminology

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API changes

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Data model changes

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Release notes snippet

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