Upgrade Airflow to 2.6

[antidipyramid]

Overview

This PR upgrades Airflow to the latest version (2.6.0).

Checklist

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

Notes

A couple of things not reflected in the title and/or code:

• I changed the way our deployment configs are organized so that they're less annoying to deal with. I also moved deployment secrets into an encrypted .env file and decrypted the Supervisor settings.
• I upgraded Python on the staging server following these instructions: https://cloudbytes.dev/snippets/upgrade-python-to-latest-version-on-ubuntu-linux
• Contrary to @antidipyramid's surgical approach, I dropped the Airflow database before deploying to sidestep those upgrade issues.

Testing Instructions

These changes have been deployed to the staging site at https://la-metro-dashboard-staging.datamade.us

• Run docker-compose build
• Run docker-compose up and verify the Airflow version at localhost:8080

Handles #95

[hancush]

Followed these instructions to upgrade Python on the server: https://cloudbytes.dev/snippets/upgrade-python-to-latest-version-on-ubuntu-linux

[hancush]

@xmedr @antidipyramid Looks like things are running smoothly on staging! Can y'all take a look at this PR next week?

[antidipyramid]

@hancush I've been able to run DAGs locally as expected using Airflow 2.6. Everything seems to be working!