feat(opendesk-mariadb): Added support for secrets

BREAKING CHANGE: Environment variable MARIADB_ROOT_PASSWORD is now propagated with the help of a Kubernetes secret instead of a plain value

Summary:

• Added support for the usage of a Kubernetes secret for mariadb.rootPassword
• Adjusted the registry of the bitnami/common chart to registry.opencode.de/...
• Adjusted the copyright information in files affected by this MR
• Added comments to relevant keys in the values.yaml file used by the README-auto-generation