[terraform] enable debug logs by default on staging
Summary: this might help with investigating https://linear.app/comm/issue/ENG-7965/keyserver-not-uploading-new-one-time-keys and future issues
Test Plan: @will is using staging right now. when he's done, i can apply this terraform change and confirm that debug logs are visible in cloudwatch
Reviewers: will
Reviewed By: will
Subscribers: ashoat, tomek, will
Differential Revision: https://phab.comm.dev/D11783