[15.0] Remove Temporary container for 3.9 python, set python 3.9 as default gemnasium-python analyzer

Proposal

Following the deprecation and removal announcement for python 3.9, we now need to:

• remove the CI configuration that builds this image
• switch default version of python to 3.9 instead of 3.6
• update tests

NB: release post announcement is handled in a separate issue: #351503 (closed)

Implementation plan

• update .gitlab-ci.yml
  • set 3.9 as default
    • set DS_PYTHON_VERSION to 3.9 in dockerfile: https://gitlab.com/gitlab-org/security-products/analyzers/gemnasium-python/-/blob/master/Dockerfile#L1
    • set .functional DS_PYTHON_VERSION to 3.9 in .gitlab-ci.yml: https://gitlab.com/gitlab-org/security-products/analyzers/gemnasium-python/-/blob/master/.gitlab-ci.yml#L116
    • update comments and other mentions of 3.6
  • change 3.9 image builds and integration tests to 3.10
    • change base .python-3.9-version job to .python-3.10-version and its DS_PYTHON_VERSION https://gitlab.com/gitlab-org/security-products/analyzers/gemnasium-python/-/blob/master/.gitlab-ci.yml#L14
    • update jobs relying on .python-3.9-version job above
    • update specs to change 3.9 tag and version to 3.10
    • update comments mentioning 3.9
• update documentation to change language version https://docs.gitlab.com/ee/user/application_security/dependency_scanning/#supported-languages-and-package-managers
• ensure the deprecated gemnasium-python:2 image (using python 3.6) produces a report that passes the schema validation, which will be enforced in GitLab 15.0

This is a breaking change and should be released with the next Major version of the analyzer See #353974 (closed)

Testing

Integration tests and secure test projects to be updated