Bump ubuntu from f8f6584 to c9820a4 in /dev/ci/docker_linux (#130292)

Bumps ubuntu from `f8f6584` to `c9820a4`.

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dev/ci/docker_linux/Dockerfile

@@ -12,7 +12,7 @@
 
 # Last manual update 2021-09-24 (changing this comment will re-build image)
 
-FROM ubuntu:focal@sha256:f8f658407c35733471596f25fdb4ed748b80e545ab57e84efbdb1dbbb01bd70e
+FROM ubuntu:focal@sha256:c9820a44b950956a790c354700c1166a7ec648bc0d215fa438d3a339812f1d01
 MAINTAINER Flutter Developers <flutter-dev@googlegroups.com>
 
 ENV TZ=America/Los_Angeles