fix: inconsistency in env var usage

[estellebertrand]

Correct some inconsistencies in the environment variables :

• old variables from Airflow 2 corrected to 3
• ensuring env variables are used whenever possible to avoid errors if the user changes some variable values

Changes in docker-compose :

• Replace hardcoded AIRFLOW__CELERY__RESULT_BACKEND with one containing variables
• Let other mentions of value from POSTGRES_... variables use the actual env variables

Changes in .env.example :

• Move from old variable name AIRFLOW__CORE__SQL_ALCHEMY_CONN to Airflow 3 one AIRFLOW__DATABASE__SQL_ALCHEMY_CONN
• Remove old Airflow login variables to match those used in the docker-compose : _AIRFLOW_WWW_USER_USERNAME and
  _AIRFLOW_WWW_USER_PASSWORD
• Let the user defines its own value in POSTGRES_... variables

Note: GitGuardian raised an alarm for the default values used airflow as login and secret (even though if it was already the default before for testing purpose) - alarm has been skipped since it's not a real secret but showcase value

[gitguardian]

⚠️ GitGuardian has uncovered 1 secret following the scan of your pull request.

Please consider investigating the findings and remediating the incidents. Failure to do so may lead to compromising the associated services or software components.

🔎 Detected hardcoded secret in your pull request

GitGuardian id	GitGuardian status	Secret	Commit	Filename
29418647	Triggered	Generic Password	0640d10	docker-compose.yml	View secret

🛠 Guidelines to remediate hardcoded secrets

1. Understand the implications of revoking this secret by investigating where it is used in your code.
2. Replace and store your secret safely. Learn here the best practices.
3. Revoke and rotate this secret.
4. If possible, rewrite git history. Rewriting git history is not a trivial act. You might completely break other contributing developers' workflow and you risk accidentally deleting legitimate data.

To avoid such incidents in the future consider

• following these best practices for managing and storing secrets including API keys and other credentials
• install secret detection on pre-commit to catch secret before it leaves your machine and ease remediation.

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^{🦉 GitGuardian detects secrets in your source code to help developers and security teams secure the modern development process. You are seeing this because you or someone else with access to this repository has authorized GitGuardian to scan your pull request.}

[Pierlou]

I have not checked but if some variables are Airflow 3-only compatible, we should rather modify them in the dedicated PR

[estellebertrand]

The merge will solve that by directly adding my commits to the open PR you mention !