Transition repository to GitHub Actions/GitLab CI

[Eder-K]

We want to eventually have this repo move away entirely from TravisCI and have everything run on either GitHub Actions/Gitlab CI. In order to preserve all functionalities, several changes need to made to multiple files. Here a rough outline of what needs to be done:

• port the .travis.yml file to its equivalent on the new platform. We can split up the original .travis.yml file and set up multiple workflows here, for example differentiate cron jobs, regular builds for systemtest PRs, and specific builds that get triggered through PRs on adapter repositories. We can also possibly set up a workflow to build the solver images (currently this is done manually on local machines, since updates to the solver images are very rare).

• update the trigger_systemtest.py and push.py scripts, which currently use the Travis API to send requests. Depending on how well we are able to make PR builds work in the point above, these scripts might not be necessary anymore.

• optionally, we can overhaul the docker image pushing system that we currently use. This would fit more under 'Refactoring the docker image system altogether', but I want to mention it here since the transition changes might overlap with some goals of the refactoring. In any case, the refactoring should be discussed in further detail in the related issue and PR.