Reverts Java 15 features from a backport to 2.x

[brianf-aws]

2.x branch compiles Java 11 code thus any features subsequent of this version will have issues

Description

used string literals. And used get(0) instead of getFirst(). and on streams used .collect(Collectors.toList())

Testing

• /gradlew test <-- where the issue could be found initially via the UT that had the Java 15 feature
• ./gradlew spotlessApply

Related Issues

Resolves #1117

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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[ryanbogan]

I think you need to add Locale.ROOT as the first argument in String.format to pass the forbidden APIs check.

i.e.
https://github.com/opensearch-project/neural-search/blob/main/src/main/java/org/opensearch/neuralsearch/search/query/HybridCollectorManager.java#L319-L323

[brianf-aws]

I think you need to add Locale.ROOT as the first argument in String.format to pass the forbidden APIs check.

Hey @ryanbogan thanks for letting me know I guess I'm forming a bad habit of not running build and just checking whether it passes UT and compiles.