gitlab-ci: Extend filtering by package to all retrieval jobs

Currently FILTER only works for GitLab projects, the OBS and APT jobs always retrieve everything, which is particularly costly for OBS as it scans the content of each package.

Drop the FILTER variable and replace it with a PROJECTS_NAMESPACE variable for GitLab and a FILTER_PACKAGES variable to be used by all jobs to narrow the amount of data retrieved, which is often very useful for testing.

Test filtering for dash:

• pipeline: https://gitlab.apertis.org/infrastructure/dashboard/-/pipelines/591183
• rendered: https://infrastructure.pages.apertis.org/-/dashboard/-/jobs/2625252/artifacts/public/index.html
• compare to: https://infrastructure.pages.apertis.org/dashboard/#pkg-dash