Attaching Theme Library to Custom Module
Based on that structure, you're referencing the library incorrectly.
It should be $form['#attached']['library'][] = 'bootstrap_subtheme/bootstrap_subtheme.global-styling';
As noted in a comment below, the duplicate use of the theme name may not be necessary, and $form['#attached']['library'][] = 'bootstrap_subtheme/global-styling'; should work according to the docs. (Note the use of a dash, matching the library name versus an underscore in your code.)