Remove deprecated `started` state for iteration

Why are we doing this work

For iterations current is a more descriptive name than started which is used in milestones. So we would like to align the status of the iteration with the actual name we are using.

Given that started status name for the iteration is also exposed to the user through parameter values in REST anf GraphQL API we will need to follow a deprecation process, besides just renaming the state.

Relevant links

This was also suggested in #328457 (comment 584848820)

Implementation plan

• Deprecate started status, milestone 14.1, !62946 (merged)
• Remove the deprecation, milestone 14.6
• Announce the deprecation 14.8, !79006 (merged)

Availability and Testing

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