Message migration message never shown

Note: Test on Clean and Boost

1. Install a fresh 3.4 test site with at least three users (A, B and C).
   • E.g. using mdk:
     1. Uninstall your current site: mdk uninstall
     2. Checkout Moodle 3.4: git checkout v3.4.0
     3. Cleanup folders that may have been left by plugins that were installed in 3.5 and up: "rm -rf blocks/timeline blocks/recentlyaccesseditems repository/nextcloud mod/lti/service/gradebookservices admin/tool/dataprivacy admin/tool/policy"
     4. Install: mdk install -r users
2. Log in as A.
3. Click on the messaging icon in the top right.
4. Click on 'See all'.
5. Search for and send a message to user B.
6. Log in as C.
7. Click on the messaging icon in the top right.
8. Click on 'See all'.
9. Search for and send a message to user B.
10. Log out.
11. Upgrade your site.
    • If testing on master: git checkout master, then proceed with the upgrade
    • If testing on 36: git checkout MOODLE_36_STABLE, then proceed with the upgrade
12. Log in as user A.
13. Confirm on the messaging drawer there is a message saying that your messages have not been migrated.
14. Run the cron.
15. Confirm this message has gone and you can see your conversation with B.
16. Log in as B.
17. Confirm on the messaging drawer there is a message saying that your messages have not been migrated and you can see the conversation with A.
18. Run the cron.
19. Confirm this message has gone and you can see your conversation with A and C.