Update to v1.26.0

https://github.com/research-software-directory/RSD-as-a-service/releases/tag/v1.26.0

Blocked by #151 (closed)

Breaking changes This release updates the PostgreSQL database to version 15. Note that additional migration steps are needed when updating from 1.24 to 1.25. These steps are described in detail here: https://github.com/research-software-directory/RSD-production/blob/main/database-migration/migration-scripts/1.25.0-to-1.26.0.md