[CI] Use official UBI9 image for testing (elastic#126455) (elastic#126508)

Internal UBI images from `docker.elastic.co` are planned to be removed.

Author: jozala

distribution/docker/build.gradle

@@ -409,8 +409,8 @@ void addBuildDockerImageTask(Architecture architecture, DockerBase base) {
       // is functional.
       if (base == DockerBase.IRON_BANK) {
         Map<String, String> buildArgsMap = [
-          'BASE_REGISTRY': 'docker.elastic.co',
-          'BASE_IMAGE'   : 'ubi9/ubi',
+          'BASE_REGISTRY': 'docker.io',
+          'BASE_IMAGE'   : 'redhat/ubi9',
           'BASE_TAG'     : 'latest'
         ]