Replace BSTooltip with GlTooltip in app/assets/javascripts/group_label_subscription.js

What does this MR do?

This Merge Request replaces the calls to Bootstrap’s jQuery tooltip API with a GitLab’s tooltip API implemented in #229044 (closed). This is part of the effort to replace Bootstrap’s javascript library with GitLab UI: &4383 (closed)

How to test?

(labels have already been set up in review app here: https://gitlab-review-249799-rep-bmfgp2.gitlab-review.app/test-group/test-project/-/labels, you can go to that link and jump to last step)

1. Make a group
2. Make a label at the group level
3. Make a project in the group
4. Make a label at the project level on that project
5. Go to the labels page for that project, e.g. http://0.0.0.0:3000/test-group-1/test-project-in-a-group/-/labels
6. Subscribe to the group label at the group level
7. Subscribe to the project label
8. Hover over both buttons, and make sure the "unsubscribe" tooltips appear.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines

Closes #249799 (closed)