More conversions to axios

What does this MR do?

Converts the following $.ajax to axios:

• app/assets/javascripts/mini_pipeline_graph_dropdown.js
• app/assets/javascripts/network/branch_graph.js
• app/assets/javascripts/notes.js
• app/assets/javascripts/notifications_form.js
• app/assets/javascripts/pager.js
• app/assets/javascripts/pages/admin/cohorts/usage_ping.js
• app/assets/javascripts/pages/dashboard/todos/index/todos.js

What are the relevant issue numbers?

#42397 (closed)