Upgrade Java to version 21 to fix build error

Dockerfile

@@ -1,13 +1,14 @@
 # Use Python 3.10 as base
 FROM python:3.10
 
-# 1. Install Java (REQUIRED for PySpark)
+# 1. Install Java 21 (Updated from 17)
+# The logs specifically asked for openjdk-21-jdk-headless
 RUN apt-get update && \
-    apt-get install -y openjdk-17-jre-headless && \
+    apt-get install -y openjdk-21-jdk-headless && \
     apt-get clean;
 
-# Set JAVA_HOME (Spark needs this)
-ENV JAVA_HOME=/usr/lib/jvm/java-17-openjdk-amd64
+# 2. Set JAVA_HOME to the new version
+ENV JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
 
 # Create a non-root user (Required by Hugging Face)
 RUN useradd -m -u 1000 user