Removal of API v3 from the codebase

This issue is to track our plan to remove API v3, that were deprecated in 9.0.

From a Product point of view, we should be aware if there are implications that prevent it to happen:

• Jenkins GitLab plugin is still using API v3 (#34603 (closed))
• Git Tower (see https://gitlab.com/gitlab-com/infrastructure/issues/2532#note_37793184)