Remove postgres subcharts from Airflow Helm Charts

### Body Bitnami (now VMware) has restricted access to newer postgres images in their main repository. This affects all KinD-based testing and development workflows, as temporary workaround we have used `docker.io/postgres` images: https://github.com/apache/airflow/pull/55820 but over a long term, we should remove that subchart as discussed in lazy consensus too: https://lists.apache.org/thread/3o4wj76pmd2m13jho7sxcbfkbgpv88h9 The plan of action could be: * Remove postgres dependency entirely from the helm chart (lazy consensus has been achieved: https://lists.apache.org/thread/3o4wj76pmd2m13jho7sxcbfkbgpv88h9) * Make sqlite the default database in values.yaml for development/testing * Update CI/CD pipelines to use sqlite * Eliminate bitnami dependency * Users can extend our helm charts and provide their own DBs on top of it. ### Committer - [x] I acknowledge that I am a maintainer/committer of the Apache Airflow project.