Fix deprecated code report issues for 9.3.x

Problem/Motivation

I got a Call to deprecated functions drupal_get_path() and file_create_url() running drupal-check.

 ------ -----------------------------------------------------------------------------
  Line   adminimal_theme.theme
 ------ -----------------------------------------------------------------------------
  45     Call to deprecated function drupal_get_path():
         in drupal:9.3.0 and is removed from drupal:10.0.0. Use
           \Drupal\Core\Extension\ExtensionPathResolver::getPath() instead.
  48     Call to deprecated function file_create_url():
         in drupal:9.3.0 and is removed from drupal:10.0.0.
           Use the appropriate method on \Drupal\Core\File\FileUrlGeneratorInterface
           instead.
  49     Call to deprecated function file_create_url():
         in drupal:9.3.0 and is removed from drupal:10.0.0.
           Use the appropriate method on \Drupal\Core\File\FileUrlGeneratorInterface
           instead.
 ------ -----------------------------------------------------------------------------

Steps to reproduce

$> drupal-check web/themes/contrib/adminimal_theme

 ------ -----------------------------------------------------------------------------
  Line   adminimal_theme.theme
 ------ -----------------------------------------------------------------------------
  45     Call to deprecated function drupal_get_path():
         in drupal:9.3.0 and is removed from drupal:10.0.0. Use
           \Drupal\Core\Extension\ExtensionPathResolver::getPath() instead.
  48     Call to deprecated function file_create_url():
         in drupal:9.3.0 and is removed from drupal:10.0.0.
           Use the appropriate method on \Drupal\Core\File\FileUrlGeneratorInterface
           instead.
  49     Call to deprecated function file_create_url():
         in drupal:9.3.0 and is removed from drupal:10.0.0.
           Use the appropriate method on \Drupal\Core\File\FileUrlGeneratorInterface
           instead.
 ------ -----------------------------------------------------------------------------

Proposed resolution

Fixes for:

• drupal_get_path() and drupal_get_filename() have been deprecated in favor of extension listing services https://www.drupal.org/node/2940438
• Functions file_create_url() and file_url_transform_relative() are deprecated https://www.drupal.org/node/2940031