Update cimg/openjdk Docker tag to v21.0.8 (#145)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

.circleci/config.yml

@@ -3,7 +3,7 @@ version: 2.1
 jobs:
   build:
     docker:
-      - image: 'cimg/openjdk:21.0.6'
+      - image: 'cimg/openjdk:21.0.8'
     steps:
       - checkout
       - run: