[native] [23/34] ReactNav6: Replace mode="modal" with presentation: 'modal'

Summary:
Context on change here.

Depends on D5415

Test Plan: Flow + ran the app on my phone and played around with it

Reviewers: atul, tomek, bartek

Reviewed By: atul

Subscribers: abosh

Differential Revision: https://phab.comm.dev/D5416