Update docker image for ubuntu2004_arm64 (#1911)

This also allows the Linux arm64 platform to use JDK 21

I manually built and updated the docker images on the two Works-on-arm
machines.
diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index 7b95756..04f501a 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -480,10 +480,10 @@
         "python": "python3.8",
     },
     "ubuntu2004_arm64": {
-        "name": "Ubuntu 20.04 LTS ARM64 (OpenJDK 11, gcc 9.4.0)",
-        "emoji-name": ":ubuntu: Ubuntu 20.04 LTS ARM64 (OpenJDK 11, gcc 9.4.0)",
+        "name": "Ubuntu 20.04 LTS ARM64",
+        "emoji-name": ":ubuntu: Ubuntu 20.04 LTS ARM64",
         "publish_binary": [],
-        "docker-image": f"gcr.io/{DOCKER_REGISTRY_PREFIX}/ubuntu2004-java11",
+        "docker-image": f"gcr.io/{DOCKER_REGISTRY_PREFIX}/ubuntu2004",
         "python": "python3.8",
         "queue": "arm64",
         # TODO: Re-enable always-pull if we also publish docker containers for Linux ARM64