Set up hourly monitoring for upstream compatibility

pmaports.git has upstream compatibility checks, that only run when pushing to master. It's important to have these checks, but combining them with pushes to master is not so great because if they fail, it's most of the time completely unrelated to the commit for which they failed.

There's another check, test_soname_bump, which I had disabled one month ago due to some missing library in alpine: 6b267e80

I've been reluctant to enable it again because of the above mentioned reason.

Instead, I propose that we create a new "monitoring" repository, which runs a .gitlab-ci.yml hourly (using "CI / CD" -> "Pipeline Schedules" feature). This pipeline should:

• run test_soname_bump
• report failures to the core team

For the future, it would be great to:

• also run the other upstream compat tests that we still have in the repository, the one which checks if musl-armhf needs to be updated IIRC
• possibly generate a nice badge somehow, that we can integrate into pmaports (just like the bpo status)
• add more tests if we like, the wiki test could also run hourly for example