Modernize legacy DoFn in BatchElementsExample (#40557)

[CH-Abhinav]

Issue

Fixes #40557

Description

Modernizes the LogOutput DoFn in BatchElementsExample.java to use parameter injection (@Element and OutputReceiver) instead of the legacy ProcessContext.

Changes

• Replaced ProcessContext c parameter with @Element List<String> batch and OutputReceiver<String> receiver.
• Verified with ./gradlew :examples:java:compileJava and ./gradlew :examples:java:spotlessApply.

[github-actions]

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