Update mirror.gcr.io/library/ubuntu Docker tag to v26 (#14600)

Author: renovate-bot

.kokoro/docker/Dockerfile

@@ -14,7 +14,7 @@
 
 # We want to use LTS ubuntu from our mirror because dockerhub has a
 # rate limit.
-FROM mirror.gcr.io/library/ubuntu:24.04
+FROM mirror.gcr.io/library/ubuntu:26.04
 
 ENV DEBIAN_FRONTEND noninteractive