Project iterations implementation/cleaning

Right now we have project iterations prepared in code and the db (https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/models/iteration.rb#L17) but we don't use them.

We need to clarify if we want to use them.

• Answer: Iterations will live in Namespace. Given Project is migrating to ProjectNamespace, we should do the cleaning with this context in mind. We still want to maintain the roll-down behavior and eventually iterations can and will exist at the Project level but via ProjectNamespace.

If we want to use them we need to

• change validations
• create default cadences (model change needed)

If we don't want to use them we need to

• remove them from code
• remove the column from the db table
  • Ignore the column in 15.5 !95988 (merged)
  • Drop the column after 15.6 #372125 (closed)
  • Remove ignore_column in 15.7 #372126 (closed)

Availability and Testing

Manual job package-and-qa will need to be run in the MRs that introduce the changes.