[SPARK-44302][BUILD] Reenable PySpark test on the daily test of Java 21 after the new arrow version release

### What changes were proposed in this pull request?
This PR aims to re-enable PySpark in Java 21.
This depends on apache#42181 .

### Why are the changes needed?
To have Java 21 test coverage.

### Does this PR introduce _any_ user-facing change?
No.

### How was this patch tested?
Pass GA.

### Was this patch authored or co-authored using generative AI tooling?
No.

Closes apache#42646 from panbingkun/SPARK-44302.

Authored-by: panbingkun <pbk1982@gmail.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

Author: panbingkun

.github/workflows/build_java21.yml

@@ -42,7 +42,7 @@ jobs:
       jobs: >-
         {
           "build": "true",
-          "pyspark": "false",
+          "pyspark": "true",
           "sparkr": "true",
           "tpcds-1g": "true",
           "docker-integration-tests": "true"