Grafana Admin Credentials Sourcing from PVC Instead of Secret

**Description**:

We are using DevLake's Helm chart to deploy Grafana in our Kubernetes cluster. However, despite configuring Grafana to use a pre-existing Kubernetes secret for the admin credentials, it appears that Grafana is still sourcing the admin credentials from the PVC. This is causing login issues where the password from the PVC is used instead of the password stored in the secret.

**Steps to Reproduce**:
Deployment Configuration: We have the following configuration in values.yaml:

```
persistence:
type: pvc
enabled: false
# storageClassName: default
accessModes:
- ReadWriteOnce
size: 10Gi
# annotations: {}
finalizers:
- kubernetes.io/pvc-protection

admin:
existingSecret: "devlake-grafana-secret"
userKey: admin-user
passwordKey: admin-password
```

**Secret Creation**: We have already created the secret (devlake-grafana-secret) with the following credentials:

```
kubectl create secret generic devlake-grafana-secret \
--from-literal=admin-user="admin" \
--from-literal=admin-password="yourStrongPassword"
```

**Deployment**: We then deploy or upgrade the Helm chart using the following command:

`helm upgrade --install devlake-grafana grafana/grafana -f values.yaml`
Login Attempt: After deployment, when we try to log in to Grafana using the admin user and the password (yourStrongPassword), we are unable to authenticate.

**Expected Behavior:**
- Use the admin credentials from the Kubernetes secret (devlake-grafana-secret) as configured in the values.yaml.
- Not use the credentials from the PVC, which are incorrect or outdated.

**Actual Behavior:**
Despite correctly configuring the secret in the values.yaml file, Grafana seems to be sourcing the admin credentials from the PVC, causing a failed login with the password stored in the PVC instead of the password defined in the secret.
I mentioned that it's using the PVC values because the PVC wasn’t deleted. The password currently working is the one that was set during the initial installation.

**Impact:**
Inability to login with the expected admin credentials (from the secret).
We are unable to authenticate as the admin user using the correct credentials.

**Environment:**
DevLake Version: 1.0.3-beta1