Add missing deprecation notices

[yscik]

Some methods only had a @deprecated tag with no user-facing warning. This adds the appropriate _deprecated_function calls.

• Sensei_Learner_Management::get_learner_full_name - replaced by Sensei_Learner::get_full_name
• Sensei_Frontend::sensei_get_template_part - replaced by Sensei_Templates::get_part
• Sensei_Frontend::sensei_course_image - replaced by Sensei()->course->course_image
• Sensei_Frontend::sensei_lesson_image - replaced by Sensei()->lesson->lesson_image
• Sensei_Frontend::sensei_lesson_quiz_meta - replaced by Sensei_Lesson::footer_quiz_call_to_action
• Sensei_Utils::sensei_save_quiz_answers - replaced by Sensei_Quiz::save_user_answers
• Sensei_Utils::sensei_grade_quiz_auto - replaced by Sensei_Grading::grade_quiz_auto
• Sensei_Utils::sensei_grade_question_auto - replaced by Sensei_Grading::grade_question_auto
• Sensei_Utils::sensei_get_user_question_answer_notes - replaced by Sensei()->quiz->get_user_question_feedback
• quiz_questions - replaced by Sensei_Templates::get_template
• sensei_check_prerequisite_course - replaced by Sensei_Course::is_prerequisite_complete

Testing instructions:

• Using these methods should show a user error when WP_DEBUG is on.

[gikaragia]

I think that we should use the current version for deprecation otherwise we will remove these functions too early.

[renatho]

I agree with @gkaragia about the version. And the rest LGTM!

[yscik]

@jom found that this was the way it was handled previously, like in #2423.
I'd rather not add conflicting versions, so if we use 3.0.0, it would probably be best to update the @deprecated tags too.
We can also just add these to a Remove deprecated stuff for 5.0 ticket and track it that way.

[gikaragia]

The reason that I asked for this is to make sure that it takes 2 major version bumps before removing the methods. It's totally fine if we add the changes to the 5.0 ticket and make sure that we don't remove them earlier.

[renatho]

We can also just add these to a Remove deprecated stuff for 5.0 ticket and track it that way.

I think it makes sense! Just to prevent a mistake, maybe we could also add a comment like this: https://github.com/Automattic/sensei-wc-paid-courses/blob/master/includes/woocommerce-integrations/class-sensei-wc.php#L739

WDYT?

[yscik]

Yup that's a good idea!