fix: Set FERNET_KEY to prevent connections from crashing webservers

[sbernauer]

Description

Fixes #694

Also did a bit of renaming variables to better reflect their prupose

Definition of Done Checklist

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Author

• Changes are OpenShift compatible
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Reviewer

• Code contains useful comments
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Release notes

Are in CHANGELOG.md
Current state:

• Prevent Airflow connections from breaking in combination with Airflow 3.
  This was achieved by setting the AIRFLOW__CORE__FERNET_KEY env var

[adwk67]

LGTM. Have started a test run here: https://testing.stackable.tech/view/02%20Operator%20Tests%20(custom)/job/airflow-operator-it-custom/61/

2 test failures: both in remote-logging when attempting to set up minio...