Bumps ubuntu from `f8f6584` to `c9820a4`.

[![Dependabot compatibility score](https://dependabot-badges.githubapp.com/badges/compatibility_score?dependency-name=ubuntu&package-manager=docker&previous-version=focal&new-version=focal)](https://docs.github.com/en/github/managing-security-vulnerabilities/about-dependabot-security-updates#about-compatibility-scores)

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