Change state to integer for issuables, remove state column - Release 3 of 3

This is the third step of the plan to deprecate state column for issues and merge requests.
We should remove the state column from issues and merge_requests table using a migration.

Schedule

First release: https://gitlab.com/gitlab-org/gitlab-ce/issues/51789

1. Add a state_id column to issues and merge requests tables

2. Schedule a background migration that will populate state_id column with new values

Second release: #28457 (closed)

1. Adds new indexes to state_id column

2. Add the code which uses the new state_id column

Third release: This issue

1. Markesstate column as ignored for issues and merge requests

Fourth release: #36370 (closed)

1. Removes state column from issues and merge_requests with database migration.