[terraform] Use JSON logs in Blob Cleanup task

Summary: We have separate task definition for blob cleanup. Now, when Blob Service supports JSON logs, we can enable them for this task too

Test Plan: terraform plan. I'll check logs format after this is landed by browsing the /ecs/blob-cleanup log group.

Reviewers: will

Reviewed By: will

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13444