Remove the term "on-call" to avoid confusion

This changes the phrase pipeline on-call rotation to pipeline triage rotation.

The rotation is not truly an on-call rotation. Using that term could mislead applicants into thinking they'd be required to be paged while off duty, possibly including night shifts. That isn't the case so we shouldn't use the term "on-call".

See the discussion in https://gitlab.com/gitlab-org/quality/team-tasks/issues/259#note_246306275