Update CI to use jdk-21

This updates the CI system to use jdk-21, which is latest LTS supported version.
Coming from opensearch-project/OpenSearch#10334

Signed-off-by: John Mazanec <jmazane@amazon.com>

.github/workflows/CI.yml

@@ -20,7 +20,7 @@ jobs:
   Build-k-NN-Linux:
     strategy:
       matrix:
-        java: [11, 17]
+        java: [11, 17, 21]
 
     name: Build and Test k-NN Plugin on Linux
     runs-on: ubuntu-latest
@@ -57,7 +57,7 @@ jobs:
   Build-k-NN-MacOS:
     strategy:
       matrix:
-        java: [ 11, 17 ]
+        java: [ 11, 17, 21 ]
 
     name: Build and Test k-NN Plugin on MacOS
     needs: Get-CI-Image-Tag
@@ -84,7 +84,7 @@ jobs:
   Build-k-NN-Windows:
     strategy:
       matrix:
-        java: [ 11, 17 ]
+        java: [ 11, 17, 21 ]
 
     name: Build and Test k-NN Plugin on Windows
     needs: Get-CI-Image-Tag

.github/workflows/backwards_compatibility_tests_workflow.yml

@@ -13,7 +13,7 @@ jobs:
   Restart-Upgrade-BWCTests-k-NN:
     strategy:
       matrix:
-        java: [ 11, 17 ]
+        java: [ 11, 17, 21 ]
         os: [ubuntu-latest]
         bwc_version : [ "2.0.1", "2.1.0", "2.2.1", "2.3.0", "2.4.1", "2.5.0", "2.6.0", "2.7.0", "2.8.0", "2.9.0", "2.10.0", "2.11.0-SNAPSHOT"]
         opensearch_version : [ "3.0.0-SNAPSHOT" ]
@@ -86,7 +86,7 @@ jobs:
   Rolling-Upgrade-BWCTests-k-NN:
     strategy:
       matrix:
-        java: [ 11, 17 ]
+        java: [ 11, 17, 21 ]
         os: [ubuntu-latest]
         bwc_version: [ "2.11.0-SNAPSHOT" ]
         opensearch_version: [ "3.0.0-SNAPSHOT" ]

.github/workflows/test_security.yml

@@ -15,7 +15,7 @@ jobs:
   Build-ad:
     strategy:
       matrix:
-        java: [ 11,17 ]
+        java: [ 11,17,21 ]
         os: [ubuntu-latest]
       fail-fast: true