Replace Ubuntu 16.04 LTS with Ubuntu 18.04 LTS in the GitLab CI (v9.11)

Ubuntu 16.04 (Xenial Xerus) is reaching End of Standard Support in April 2021 thus we are removing it from the list of supported platforms and replacing it with Ubuntu 18.04 LTS (Bionic Beaver).

(cherry picked from commit 4402a90b)

Edited by Ondřej Surý

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