Fix. db:migrate is failed on MySQL 8

```
StandardError: An error has occurred, all later migrations canceled:

Mysql2::Error: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'system = true AND note LIKE '\_%\_'' at line 1: UPDATE notes SET note = trim(both '_' from note) WHERE system = true AND note LIKE '\_%\_'
```

changelogs/unreleased/fix-db-migrate-is-failed-on-mysql8.yml

+---
+title: Fix. `db:migrate` is failed on MySQL 8
+merge_request: 28351
+author: sue445
+type: fixed

db/migrate/20150509180749_convert_legacy_reference_notes.rb

@@ -7,7 +7,8 @@
 #   mentioned in 54f7727c850972f0401c1312a7c4a6a380de5666
 class ConvertLegacyReferenceNotes < ActiveRecord::Migration[4.2]
   def up
-    execute %q{UPDATE notes SET note = trim(both '_' from note) WHERE system = true AND note LIKE '\_%\_'}
+    quoted_column_name = ActiveRecord::Base.connection.quote_column_name('system')
+    execute %Q{UPDATE notes SET note = trim(both '_' from note) WHERE #{quoted_column_name} = true AND note LIKE '\_%\_'}
   end
 
   def down