Mentions-related warnings in browser when upgrading XWiki

STEPS TO REPRODUCE

1. Create (or unzip) an instance of XWiki 12.6.7
2. Close the instance
3. Go to a newer instance (e.g. 12.10.2) and copy/cut the

   [...]webapps\xwiki

   folder and replace it (first delete the old one) in the same location on the older (12.6.7) version

1. Start XWiki and follow DW

EXPECTED RESULTS

No warnings/errors are displayed in browser when running Distribution Wizard.

ACTUAL RESULTS

On 'Step 2 - Flavor', there are some warnings displayed, related to User Mentions:

The issue could not be reproduced when upgrading from XWiki 12.6.7 to XWiki 12.9.