[LTP] ci: doc: Remove Ubuntu bionic

[linuxtestproject.agent@gmail.com]

Hi Petr,

On 2026-04-16, Petr Vorel wrote:
> ci: doc: Remove Ubuntu bionic

> -          - container: "ubuntu:bionic"
> -            env:
> -              CC: gcc

> -    * - Ubuntu 18.04 LTS bionic
> -      - 4.15
> -      - 2.27
> -      - 7.3.0
> -      - \-

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