Bug: go113 Cloud Function environment prevent using OPA version after v0.29.3

[BrunoReboul]

OPA version v0.29.4 and above imports the go package fstest which appears with go 1.16beta1.

Google Cloud functions go113 runtime related to go v1.13,15 leading to the following error when building-deploying the function:
cannot find module providing package testing/fstest

Last OPA version to use without this issue is v0.29.3 released on 2021-05-28
Current OPA version is v0.33.1 released 2021-10-04

Proposed solution:

• short team: Upgrate Cloud Function runtime env to go116, currently in preview (go 1.16.7)
• long term: move out of cloud function to cloud run to remove the constraint on language / version. enabling to use go 1.17.1 (current version)