Helm Chart: Directly Start Airflow Containers

### Description Currently, the Helm chart starts Airflow components using `bash -c exec airflow some-command`. In all cases there is no actual requirement for the shell for interpolation/substitution. The Helm chart should be updated to remove the requirement for a shell and instead start the Airflow components directly (this is similar to a prior issue which changed the Redis template in the same way). ### Use case/motivation Mainly, this would enable hardened (shell-less) images (i.e. third-party images such as Docker's hardened image) to be used as a drop-in replacement with the official chart. You can currently override all the main container's commands and arguments, but you cannot override, for example, the `wait-for-airflow-migrations` init container, which means you need a custom derivative of the chart. This is a very minor change that would not result in a functional change, but would create less friction for those who require distroless/hardened images. ### Related issues _No response_ ### Are you willing to submit a PR? - [x] Yes I am willing to submit a PR! ### Code of Conduct - [x] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)