Update Java/Kotlin matrices to current versions

.circleci/config.yml

@@ -21,7 +21,7 @@ commands:
       - sdkman/setup-sdkman
       - sdkman/sdkman-install:
           candidate: kotlin
-          version: 2.0.20
+          version: 2.1.10
       - run:
           name: Download dependencies
           command: ./bld download
@@ -46,11 +46,11 @@ jobs:
     steps:
       - build_and_test
 
-  bld_jdk20:
+  bld_jdk21:
     <<: *defaults
 
     docker:
-      - image: cimg/openjdk:20.0
+      - image: cimg/openjdk:21.0
 
     steps:
       - build_and_test
@@ -59,4 +59,4 @@ workflows:
   bld:
     jobs:
       - bld_jdk17
-      - bld_jdk20
+      - bld_jdk21