[docs-only] Announce Removal Temporary 3.6 and 3.9-temp container 3.9 python

Users (internal, external, anyone) of this image should watch this issue so they get a heads up when it will be removed and how to transition to the new image

Proposal

we announced deprecation in 14.8

Now we need to

• Announce the removal of the 3.9 custom container
• Add a specific image for python 3.6 (similarly to what we did for python 3.9) that will be removed in 15.3. Users who would like the removal extended should comment in issue #351503 (closed) .
• This will be a breaking change for the gemnasium-python analyzer, the major will have to be bumped.