[services] Backup - Fix dynamoDB item size limit

Summary:
Depends on D4323

linear task: https://linear.app/comm/issue/ENG-1288/properly-handle-the-dynamodb-item-size-limit

Finally, the proper value that is 400KiB (400 * 1024) (not KB) based on the research described in the linked task.

Test Plan: -

Reviewers: palys-swm, ashoat, varun

Reviewed By: palys-swm, ashoat

Subscribers: ashoat, palys-swm, Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D4324