- Replace Boostrap dropdown with GlDropdown for comment submit button.

- Remove note_dropdown qa selector

- Update the shared example Thread Comments
The shared examples under "Thread Comments" have split into two:
"Thread Comments for commit and snippet"
and "Thread Comments for issue, epic and merge request"

Why?
After we've replaced Bootstrap dropdown button
with GitLab UI counterpart in comment_form.vue,
issue, epic and merge request uses GitLab UI GlDropdown
that has slightly different selectors.

Previously quarantined examples have been restored in this commit:
https://gitlab.com/gitlab-org/gitlab/-/issues/196825