Add lock_version to merge_requests table

Closes #21465

It appears this column was mistakenly omitted in !5623 due to rebasing
issues in earlier comments.

db/schema.rb

@@ -620,6 +620,7 @@ ActiveRecord::Schema.define(version: 20160823081327) do
     t.string   "merge_commit_sha"
     t.datetime "deleted_at"
     t.string   "in_progress_merge_commit_sha"
+    t.integer  "lock_version"
   end
 
   add_index "merge_requests", ["assignee_id"], name: "index_merge_requests_on_assignee_id", using: :btree