Guard against existence of project_features.requirement_access_level

Adds a check whether the project_features.requirements_access_level column exists before attempting to add/remove it in the migration for !46532 (merged)

A few issues were experienced in GDK (likely because of incorrectly reverted MR !34420 (merged))

StandardError: An error has occurred, this and all later migrations canceled:
PG::DuplicateColumn: ERROR:  column "requirements_access_level" of relation "project_features" already exists

sion of GitLab depends on gitlab-shell 13.12.0, but you're running 13.11.0. Please update gitlab-shell.
== 20201030092151 AddRequirementsAccessLevelToProjectFeatures: migrating ======
-- add_column(:project_features, :requirements_access_level, :integer, {:default=>20, :null=>false})
rake aborted!
StandardError: An error has occurred, this and all later migrations canceled:

/cc @stanhu @ealcantara