Issue #3528007 by megakeegman, dydave: DeprecationHelper class not available before Drupal 10.2

https://www.drupal.org/project/admin_toolbar/issues/3528007

This is a follow-up to MR !136 (merged):

Fixed regression change breaking compatibility with Core versions lower than 10.2:

Revert "Issue #3518123 by dydave: Refactored method 'flushJsCss' with 'backwardsCompatibleCall' to support deprecated '_drupal_flush_css_js' for versions below core:10.2."

This reverts commit 9b8a258c.

Closes #3528007