[Maintenance] - Save Gantt Changes Modal Misplaced

[cielbellerose]

Description

after creating a work package on the gantt chart, the modal popup for confirming the change is at the bottom right of the screen. We want this centered on the page like other modals

Acceptance Criteria

• modal centered on the screen

Proposed Solution

frontend/src/pages/GanttPage/ProjectGanttChart/ProjectGanttChangeModals/GanttTimeLineChangeModal.tsx

• currently uses NERDraggableFormModal, we should look into if this is necessary or if the regular ner modal works better here

Mocks