Update cimg/openjdk Docker tag to v21.0.8-browsers

Author: renovate[bot]

.circleci/config.yml

@@ -22,7 +22,7 @@ jobs:
   webtests:
     <<: *defaults
     docker:
-      - image: cimg/openjdk:21.0.6-browsers
+      - image: cimg/openjdk:21.0.8-browsers
     environment:
       DISPLAY: :99