Is your feature request related to a problem? Please describe.
The JDK-21 is already out and becomes the next LTS release.

Describe the solution you'd like
We need to make sure the ecosystem is ready to be using JDK-21 at runtime as well as build time:

• Support JDK-21 builds on Build and CI opensearch-build#4071
• Update Gradle to 8.4 #10138

  New features include: initial support for JDK 21 only to compile, test, and run Gradle projects since Kotlin does not yet support JDK 21;

• Update bundled JDK to JDK-21 #10591
• Add Java 11/17/21 matrix for precommit and assemble checks #11035
• [Backport] [2.x] Update bundled JDK to JDK-21.0.1 (#10576) #11003
• [Backport 2.x] Add Java 11/17/21 matrix for precommit and assemble checks #11045

Plugins / clients / tools:

• Add Java 11/17/21 matrix for build, test and integration (main only) checks opensearch-java#705
  • [Backport 2.x] Add Java 11/17/21 matrix for build, test and integration (main only) checks opensearch-java#707
• Update to Gradle 8.4 opensearch-sdk-java#962
• security
  • Add Java 11/17/21 matrix for plugin install, test and integration test checks security#3641
  • Update to Gradle 8.4 security#3638
  • [Backport] [2.x] Add Java 11/17/21 matrix for plugin install, test and integration test checks (#3641) security#3715
  • [Backport] [2.x] Update to Gradle 8.4 (#3638) security#3640
• custom-codecs
  • Onboard jenkins prod docker images to github actions custom-codecs#76
  • Update to Gradle 8.4 custom-codecs#78
• custom-codecs
• data-prepper
  • Add Java 11/17/21 matrix for build, test and performance test checks data-prepper#3622
• https://github.com/opensearch-project/neural-search
  • Added support for jdk-21 neural-search#500
• https://github.com/opensearch-project/ml-commons
  • Update Gradle to 8.4 ml-commons#1612
  • Update Lombok and Gradle plugins to support JDK-21 ml-commons#1698
  • [Backport] [2.x] Update Gradle to 8.4 (#1612) ml-commons#1697
  • [Backport] [2.x] Bump Mockito dependencies ml-commons#1868
• https://github.com/opensearch-project/sql
  • Update to Gradle 8.4 sql#2433
  • [Backport] [2.x] Update to Gradle 8.4 (#2433) sql#2480
  • Add JDK-21 to GA worklflows sql#2481
• https://github.com/opensearch-project/security-analytics
  • Update to Gradle 8.5 security-analytics#720
• https://github.com/opensearch-project/k-NN
  • Upgrade gradle to 8.4 k-NN#1289
  • Update CI to use jdk-21 k-NN#1288
• https://github.com/opensearch-project/geospatial
  • Update CI to use jdk-21 geospatial#599
  • Upgrade gradle to 8.4 geospatial#596
• https://github.com/opensearch-project/index-management
  • Update to Gradle 8.5 index-management#1069
  • [Backport] [2.x] Update to Gradle 8.5 (#1069) index-management#1070
• https://github.com/opensearch-project/anomaly-detection
  • Update to Gradle 8.5 anomaly-detection#1131
  • [Backport] [2.x] Update to Gradle 8.5 (#1131) anomaly-detection#1132
• https://github.com/opensearch-project/notifications
  • Update Gradle to 8.5 notifications#824
  • [Backport 2.x] Update Gradle to 8.5 notifications#825
• https://github.com/opensearch-project/opensearch-hadoop
  • Update Gradle to 8.5 opensearch-hadoop#377
• https://github.com/opensearch-project/search-processor
  • Upgrade to Gradle 8.5 search-processor#213
• https://github.com/opensearch-project/alerting
  • Update to Gradle 8.5 alerting#1369
  • [Backport] [2.x] Update to Gradle 8.5 (#1369) alerting#1371
• https://github.com/opensearch-project/job-scheduler
  • Update to Gradle 8.4 job-scheduler#542
  • Gradle Update to 8.5 job-scheduler#545
  • [Backport 2.x] Gradle Update to 8.5 job-scheduler#546
• https://github.com/opensearch-project/asynchronous-search
  • Update to Gradle 8.5 asynchronous-search#516
  • Update to Gradle 8.5 asynchronous-search#519
  • [Backport 2.x] Updates admin credentials used in github workflow and Update to Gradle 8.5 asynchronous-search#522
• https://github.com/opensearch-project/common-utils
  • Update Gradle to 8.4 common-utils#560
  • Add Java 11/17/21 matrix for build, test and integration checks common-utils#561
  • [Backport 2.x] Add Java 11/17/21 matrix for build, test and integration checks common-utils#564
  • [Backport] [2.x] Update Gradle to 8.4 (#560) common-utils#563
• https://github.com/opensearch-project/performance-analyzer-commons
  • Update to Gradle 8.5 performance-analyzer-commons#72
• https://github.com/opensearch-project/opensearch-learning-to-rank-base
  • Update to Gradle 8.5 (and OpenSearch 2.11.1 baseline) opensearch-learning-to-rank-base#28
• https://github.com/opensearch-project/performance-analyzer
  • Update to Gradle 8.5 and Fixing CVE-2023-33202 performance-analyzer#617
  • [Backport] [2.x] Update to Gradle 8.5 (#617) performance-analyzer#619
  • Update spotless to meet JDK-21 baseline performance-analyzer#618
• https://github.com/opensearch-project/performance-analyzer-rca
  • Update to Gradle 8.5 performance-analyzer-rca#528
  • [Backport] [2.x] Update to Gradle 8.5 (#528) performance-analyzer-rca#535
  • Update spotless to meet JDK-21 baseline performance-analyzer-rca#533
• https://github.com/opensearch-project/opensearch-oci-object-storage
  • Update to Gradle 8.5 opensearch-oci-object-storage#58
• https://github.com/opensearch-project/sql-jdbc
  • Update to Gradle 8.5 sql-jdbc#120
• https://github.com/opensearch-project/flow-framework/
  • Update to Gradle 8.5 flow-framework#227
  • [Backport 2.x] Update to Gradle 8.5 flow-framework#247
• https://github.com/opensearch-project/opensearch-plugin-template-java/
  • Update to Gradle 8.5 opensearch-plugin-template-java#63
• https://github.com/opensearch-project/observability
  • upgrade gradle to 8.5 observability#1777
  • [Backport 2.x] upgrade gradle to 8.5 observability#1779
• https://github.com/opensearch-project/reporting
  • upgrade gradle to 8.5 reporting#941
  • [Backport 2.x] upgrade gradle to 8.5 reporting#942

There are some foreseeing delays unfortunately with Eclipse Temurin JDK-21 distribution:

• https://adoptium.net/blog/2023/09/temurin21-delay/
• September 2023 Release Status per Platform, Version & Binary Type adoptium/temurin#8

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context

• [META]: SIMD adoption in OpenSearch #9423