Remove support for JDK 8

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Because Apache Lucene 9 requires JDK 11 or newer, and coming from opensearch-project/opensearch-plugins#110, remove support for JDK 8.

• Remove JDK 8 from CI/CD, default to 11.
• Set Java source and target compatibility in all build.gradle files to JavaVersion.VERSION_11.
• Update DEVELOPER_GUIDE and remove any other mention of JDK 8.