Bump ubuntu from b795f8e to f8f6584 in /dev/ci/docker_linux

Bumps ubuntu from `b795f8e` to `f8f6584`.

---
updated-dependencies:
- dependency-name: ubuntu
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

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