[terraform] Increase Identity memory to 2048

Summary:

• Updated launch template to t3.xlarge (16GB RAM) - can consider also t3.large (8GB), I'll be experimenting with it on staging
• Updated Identity task definition to use 2048 MB memory

Test Plan: Applied on staging, then prod.

Reviewers: will, ashoat, bartek

Reviewed By: will, ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12792