[terraform] Bump staging Blob to 1.2.0

Summary:
Includes new /holders endpoints and base64 support: stack of D13479 .. D13482 and D13503
Bumped semver minor because of these new endpoints
Git tag: https://github.com/CommE2E/comm/releases/tag/blob-v1.2.0

Test Plan: terraform plan

Reviewers: will, varun, ashoat, kamil

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13510