Remove secret from airflow helm values

[kevin-bates]

Running a scan to detect secrets persisted within the Elyra repository yielded several false positives, except one specifying a Fernet Key. Since the preceding comment describes how to generate a key value, it's fairly clear the value that is there should be replaced when a user deploys Elyra via the helm chart.

This pull request replaces the value with "my-generated-fernetKey", which self-decribes what should be done, in conjunction with the preceding comment . That comment is captured here:

  ## the fernet key used to encrypt the connections/variables in the database
  ##
  ## WARNING:
  ## - you MUST customise this value, otherwise the encryption will be somewhat pointless
  ##
  ## NOTE:
  ## - to prevent this value being stored in your values.yaml (and airflow-env ConfigMap),
  ##   consider using `airflow.extraEnv` to define it from a pre-created secret
  ##
  ## GENERATE:
  ##   python -c "from cryptography.fernet import Fernet; FERNET_KEY = Fernet.generate_key().decode(); print(FERNET_KEY)"
  ##

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