Deprecating Ubuntu 18.04 in Semaphore

As of March 31, 2024, the Ubuntu 18.04 image will be phased out, following its end of standard support on May 31, 2023. A series of brownout periods will be implemented from February 5 to March 31, 2024, where jobs using Ubuntu 18.04 will not start during specified daily intervals, gradually increasing in duration. This deprecation is part of a commitment to provide a stable and secure experience, as Ubuntu 18.04 no longer receives hardware and maintenance updates. Users are encouraged to migrate to Ubuntu 20.04, which offers improved performance and longer support until April 2025. Migration involves updating the os_image: line in the semaphore.yml file from ubuntu1804 to ubuntu2004, ensuring software requirements align with Ubuntu 20.04's stack. For additional assistance, users can contact support or refer to the migration documentation.