[docs-only] Deprecation and Removal of 3.6 python for Dependency Scanning - swap for 3.9

We are removing the support of python 3.6 as it is end of life and out of support.

You should comment on this issue if you need 3.6 support and ideally work with your account manager to include a salesforce link to help us figure out if and for how long we can extend support.

Proposal

• Announce in 14.8 the deprecation of python 3.6 with a breaking change in 15.0 to no longer be default container
  • Deprecation doc: https://docs.gitlab.com/ee/update/deprecations.html#dependency-scanning-python-39-and-36-image-deprecation
  • MR !78502 (merged)
• Announce in 15.0
  • the removal of the temporary python 3.9 container
  • the removal of active support for python 3.6, but that it is available in the deprecated gemnasium-python:2 image. This means we will only offer backport for critical security issues. Though, we will ensure this image produces report that passes the schema validation, which will be enforced in GitLab 15.0

See Removal process