Retain timelogs for deleted notes

What does this MR do?

Changes the behaviour when deleting notes from: deleting related timelogs... to nullifying the note_id

Closes #245257 (closed)

Screenshots (strongly suggested)

lee@cc-gdk-2:~/gitlab-development-kit/gitlab$ bundle exec rake db:migrate:up VERSION=20210403022952 RAILS_ENV=development
== 20210403022952 RemoveNotesDeleteCascadeTimelogs: migrating =================
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:timelogs)
   -> 0.0029s
-- execute("ALTER TABLE timelogs\nADD CONSTRAINT fk_timelogs_note_id\nFOREIGN KEY (note_id)\nREFERENCES notes (id)\nON DELETE SET NULL\nNOT VALID;\n")
   -> 0.0013s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- execute("ALTER TABLE timelogs VALIDATE CONSTRAINT fk_timelogs_note_id;")
   -> 0.0015s
-- execute("RESET ALL")
   -> 0.0005s
-- foreign_keys(:timelogs)
   -> 0.0020s
-- remove_foreign_key(:timelogs, :notes, {:column=>:note_id, :on_delete=>:cascade})
   -> 0.0026s
== 20210403022952 RemoveNotesDeleteCascadeTimelogs: migrated (0.0197s) ========

lee@cc-gdk-2:~/gitlab-development-kit/gitlab$ bundle exec rake db:migrate:down VERSION=20210403022952 RAILS_ENV=development
== 20210403022952 RemoveNotesDeleteCascadeTimelogs: reverting =================
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:timelogs)
   -> 0.0025s
-- execute("ALTER TABLE timelogs\nADD CONSTRAINT fk_8d058cd571\nFOREIGN KEY (note_id)\nREFERENCES notes (id)\nON DELETE CASCADE\nNOT VALID;\n")
   -> 0.0010s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- execute("ALTER TABLE timelogs VALIDATE CONSTRAINT fk_8d058cd571;")
   -> 0.0015s
-- execute("RESET ALL")
   -> 0.0005s
-- foreign_keys(:timelogs)
   -> 0.0019s
-- remove_foreign_key(:timelogs, :notes, {:column=>:note_id, :on_delete=>:nullify, :name=>"fk_timelogs_note_id"})
   -> 0.0025s
== 20210403022952 RemoveNotesDeleteCascadeTimelogs: reverted (0.0197s) ========

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