[terraform] restrict redis access to ecs keyserver service only

Summary:
Now that we have a keyserver security group, we can specify the keyserver security group as the restriction in our ingress rule instead of universal access to the redis instance.

Depends on D12689

Test Plan: terraform apply

Reviewers: varun, bartek

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12690