Remove python 3.6 builds - failing due to lack of base image in repo mirror
Remove builds currently failing because the base image is not found in the gcr repo mirror and pull from docker hub is hitting pull limits (https://gitlab.developers.cam.ac.uk/uis/devops/infra/dockerimages/-/pipelines/67906).
The existing images will still be accessible, but updated images will not be built.
Part of https://gitlab.developers.cam.ac.uk/uis/devops/admin/-/issues/211