[terraform] bump prod backup service image to 0.4.3

Summary: staging has been on 0.4.3 for a week and we haven't observed any issues. bumping prod to the same version

Test Plan: terraform plan

Reviewers: bartek, kamil, ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13562