[terraform] Bump RabbitMQ engine_version to 3.13.7

Summary: ENG-10277.

Test Plan:
Applied this to staging and tested two core functionalities, DMs and notifs

After deploying to prod, tested that there are no changes in terrform

Reviewers: ashoat, kamil

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D14423