MDL-43236 behat: Deprecate old steps

Correcting grammar errors and deprecating
the previous steps definitions:
- behat_data_generators::the_following_exists()
- behat_general::should_exists()
- behat_general::should_not_exists()
MDL-43236 behat: Updating references to deprecated methods
MDL-43236 behat: Step arguments normalization

Updating step definitions arguments according to the
documentation page.