chore(deps): update ubuntu docker tag to v26

Author: renovate[bot]

.cursor/Dockerfile

@@ -4,7 +4,7 @@
 # the shells the completion integration tests exercise. Repo-pinned tools
 # (declared in mise.toml) are installed after checkout by .cursor/install.sh,
 # because mise.toml is only available once the repository is cloned.
-FROM ubuntu:24.04@sha256:561618e2c15bf2397621dd04f96926663a3b5616c189cf7e38db7e82f5c538ea
+FROM ubuntu:26.04@sha256:678c6550cc43645e08669028bc177f50be4e7c5b8cca677067b1914d4afc7a03
 
 ENV DEBIAN_FRONTEND=noninteractive