Update staging db url

[antidipyramid]

Overview

We upgraded the staging DB to perform load tests and test out connection pooling. See Metro-Records/la-metro-councilmatic#1065.

Checklist

• [x] PR has a descriptive enough title to be useful in changelogs

Testing Instructions

• Run blackbox_postdeploy
• Verify that LA_METRO_DATABASE_URL in configs/staging/.env has been updated

Handles #127

[fgregg]

Approved. @antidipyramid , can you also open up an issue for transitioning management of secrets from blackbox to airflow?

[antidipyramid]

@fgregg Yes, the issue is here.