[terraform] create separate lb security groups for landing and web

Summary:
Originally used a shared lb-sg but this is a bad idea given that the disabling ingress approach for the migration script would disable traffic for the webapp and landing page, not just
the keyserver.

Depends on D12874

Test Plan: terraform apply and was able to access landing page and webapp at custom domains

Reviewers: varun, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12875