[Helm Chart] Redis and Fernet Secret using pre-install hooks instead of lookup functions

### Apache Airflow version

main (development)

### If "Other Airflow 2 version" selected, which one?

_No response_

### What happened?

Problem:

I was trying to use airflow as a child chart and noticed I couldn't upgrade my existing chart because the fernet secret would not get created.

templates/configmaps/extra-configmaps.yaml: {{- $_ := set $annotations "helm.sh/hook" "pre-install,pre-upgrade" }}
templates/secrets/extra-secrets.yaml: {{- $_ := set $annotations "helm.sh/hook" "pre-install,pre-upgrade" }}
templates/secrets/fernetkey-secret.yaml: "helm.sh/hook": "pre-install"
templates/secrets/redis-secrets.yaml:# relying on the "pre-install" hack to prevent changing randomly generated passwords,
templates/secrets/redis-secrets.yaml: "helm.sh/hook": "pre-install"
templates/secrets/redis-secrets.yaml: "helm.sh/hook": "pre-install"

There are a couple references to secrets, fernet and redis secret that are using the pre-install hook to avoid them changing during helm upgrade

I think this could be done with a lookup function instead

```
apiVersion: v1
kind: Secret
metadata:
name: my-secret
labels:
type: Opaque
data:
{{- $previousSecret := lookup "v1" "Secret" .Release.Namespace "my-secret" }}
{{- if $previousSecret }}
"mysecret": {{ $previousSecret.data.mysecret }}
{{- else if .Values.mySecret }}
"mysecret": {{ .Values.mySecret | b64enc | quote }}
{{- else }}
"mysecret": {{ randAlphaNum 32 | b64enc | quote }}
{{- end }}

```
Because it seems that this pre-install hook was done more as a workaround than anything.

### What you think should happen instead?

I think fernet secret should get created on helm upgrade if it does not exist.

### How to reproduce

Try using airflow as a child chart that already has a deployed release.

### Operating System

Fedora 40

### Versions of Apache Airflow Providers

_No response_

### Deployment

Official Apache Airflow Helm Chart

### Deployment details

_No response_

### Anything else?

_No response_

### Are you willing to submit 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)

Start with templates/secrets/fernetkey-secret.yaml and templates/secrets/redis-secrets.yaml, then compare the hook annotations in templates/configmaps/extra-configmaps.yaml and templates/secrets/extra-secrets.yaml. Reproduce an upgrade with Airflow deployed as a child chart and verify that missing Fernet secrets are created on upgrade while existing Redis and Fernet secret values are preserved.

Written by the indexing model from the issue text.