[SPARK-59939] Deprecate v1beta1 version of SparkApp and SparkCluster CRDs

### What changes were proposed in this pull request?

This PR marks the `v1beta1` version of the `SparkApplication` and `SparkCluster` CRDs as deprecated by adding `deprecated: true` to the Helm chart CRDs, and adds an item to `docs/migration_guide.md`. `v1beta1` stays served, and `v1` stays the storage version.

### Why are the changes needed?

`v1` is available since [0.5.0](https://github.com/apache/spark-kubernetes-operator/releases/tag/0.5.0) (2025-09-27), but the `v1beta1` schema in the chart is a static snapshot which is no longer updated, e.g. it lacks `spec.suspend`. To remove `v1beta1` in 2.0.0, users need a deprecation period first.

### Does this PR introduce _any_ user-facing change?

Yes, compared to the released 1.0.0. `kubectl` prints a deprecation warning for `apiVersion: spark.apache.org/v1beta1`. Existing resources keep working.

### How was this patch tested?

Passed `helm lint --strict` and `gradle :spark-operator-api:assertGeneratedCRDMatchesHelmChart`.

### Was this patch authored or co-authored using generative AI tooling?

Generated-by: Claude Sonnet 5.5

Closes #924 from dongjoon-hyun/SPARK-59939.

Authored-by: Dongjoon Hyun <dongjoon@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

Author: dongjoon-hyun

build-tools/helm/spark-kubernetes-operator/crds/sparkapplications.spark.apache.org-v1.yaml

@@ -28,6 +28,7 @@ spec:
   scope: Namespaced
   versions:
     - name: v1beta1
+      deprecated: true
       schema:
         openAPIV3Schema:
           properties:

build-tools/helm/spark-kubernetes-operator/crds/sparkclusters.spark.apache.org-v1.yaml

@@ -28,6 +28,7 @@ spec:
   scope: Namespaced
   versions:
     - name: v1beta1
+      deprecated: true
       schema:
         openAPIV3Schema:
           properties:

docs/migration_guide.md

@@ -30,6 +30,8 @@ affect building the project, CI, tests or examples.
 
 ### Kubernetes and CRDs
 
+- Since 1.1.0, `spark.apache.org/v1beta1` of the CRDs is deprecated and scheduled for removal in
+  2.0.0. Its schema is no longer updated, e.g. it lacks `spec.suspend`. Use `spark.apache.org/v1`.
 - Since 1.1.0, K8s 1.35 or newer is recommended instead of 1.34 ([SPARK-59692](https://issues.apache.org/jira/browse/SPARK-59692)).
 - Since 1.1.0, the CRDs have `spec.suspend` for `SparkApplication` and `SparkCluster`, and the
   `Suspended` state for `SparkCluster`. Since `helm upgrade` does not update CRDs, replace them