T270158: [12h] Decide on the "secondary" CI for Wikibase code repository created an ADR, now we must implement the ADR for secondary CI in WIkibase.
Acceptance Criteria:
- Wikibase secondary CI no longer uses travis
- Wikibase secondary CI uses github actions
- The same matrix of jobs is run with at least email notifications
- No longer needed travis references are cleaned up in Wikibase.git