[SPARK-52676] Use Java 21 for SparkApp benchmark

[dongjoon-hyun]

What changes were proposed in this pull request?

This PR aims to use Java 21 for SparkApp benchmark.

Why are the changes needed?

BEFORE

$ docker run -it --rm apache/spark:4.0.0 java --version
openjdk 17.0.15 2025-04-15
OpenJDK Runtime Environment Temurin-17.0.15+6 (build 17.0.15+6)

AFTER

$ docker run -it --rm apache/spark:4.0.0-java21-scala java --version
openjdk 21.0.7 2025-04-15 LTS
OpenJDK Runtime Environment Temurin-21.0.7+6 (build 21.0.7+6-LTS)
OpenJDK 64-Bit Server VM Temurin-21.0.7+6 (build 21.0.7+6-LTS, mixed mode, sharing)

In addition, this PR uses the smaller image because we don't need Python.

$ docker images | grep apache/spark
apache/spark      4.0.0                                      d25d55f20f67   9 days ago      1.25GB
apache/spark      4.0.0-java21-scala                         94b89e5b436d   9 days ago      977MB

Does this PR introduce any user-facing change?

No, this is a test change.

How was this patch tested?

Manual review.

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

No.

[dongjoon-hyun]

Thank you, @viirya . Merged to main.

cc @peter-toth