Fix Helm schema URL referencing defunct kubernetesjsonschema.dev (#33135)

## Summary

This PR fixes the Helm chart schema validation failure caused by the
defunct `kubernetesjsonschema.dev` service.

### Problem

When running Helm commands (e.g., `helm template`, `helm install`),
users encounter the following error:

```
failing loading "https://kubernetesjsonschema.dev/v1.18.0/_definitions.json": HTTP request to https://kubernetesjsonschema.dev/v1.18.0/_definitions.json returned status 404 (Not Found)
```

The `kubernetesjsonschema.dev` service has been discontinued and no
longer serves JSON schema files.

### Verification

| URL | Status |
|-----|--------|
| https://kubernetesjsonschema.dev/v1.18.0/_definitions.json | ❌ 404 |
|
https://raw.githubusercontent.com/yannh/kubernetes-json-schema/master/v1.19.0/_definitions.json
| ✅ 200 |

### Solution

- Updated the schema URL from `kubernetesjsonschema.dev` to the actively
maintained GitHub repository `yannh/kubernetes-json-schema`
- Bumped the Kubernetes schema version from `1.18.0` to `1.19.0` (the
earliest version available in the replacement repository)
- Regenerated the `values.schema.json` files for both the main Dagster
chart and the user-deployments subchart

Fixes #33134

---------

Co-authored-by: Daniel Gibson <daniel@elementl.com>

Author: kang8

helm/dagster/charts/dagster-user-deployments/values.schema.json

@@ -9,7 +9,7 @@
 
 
 class SupportedKubernetes(str, Enum):
-    V1_18 = "1.18.0"
+    V1_19 = "1.19.0"
 
 
 class ConfigurableClass(PydanticBaseModel, extra="forbid"):
@@ -44,10 +44,8 @@ def schema_extra(schema, model):
                     value["anyOf"].append({"type": "null"})
 
 
-def create_definition_ref(definition: str, version: str = SupportedKubernetes.V1_18.value) -> str:
-    return (
-        f"https://kubernetesjsonschema.dev/v{version}/_definitions.json#/definitions/{definition}"
-    )
+def create_definition_ref(definition: str, version: str = SupportedKubernetes.V1_19.value) -> str:
+    return f"https://raw.githubusercontent.com/yannh/kubernetes-json-schema/master/v{version}/_definitions.json#/definitions/{definition}"
 
 
 def create_json_schema_conditionals(