Fix ci

Mariadb: See

• https://github.com/MariaDB/mariadb-docker/issues/434
• https://mariadb.org/new-service-quay-io-mariadb-foundation-mariadb-devel/ for discussion

Edit: Cant use this container because it requires a newer gitlab: https://gitlab.com/gitlab-org/gitlab/-/issues/301197 😞

Mamba:

• updated to mamba docker

Mkdocs:

• include dep https://mkdocstrings.github.io/handlers/overview/, we currently use python-legacy => supports marshmallow classes via pytkdocs
• remove heading_level, seems to no longer be supported by pytkdocs