Behaviours set read-only for multi-user pickers no longer works from Jira 8.21.0

Steps to reproduce:

1. Create a multi-user picker
2. Create an issue and add some users to that field
3. Create a field-level behaviour for that field and enable the read-only toggle
4. Save the behaviour and map it to the appropriate project
5. Go back to the issue view page and try to edit your created issue

Expected result:

You should not be able to delete users from the read-only multi-user picker, as there should be no delete button for each selected user option.

Actual result:

You can delete at least one of the users from the multi-user picker field before we remove the delete option for each selected user option.

Note:

This only impacts Jira 8.21.0 onwards as this is the version in which Jira changed the Multi-user pickers render style to use an aui-list instead of just a plain text area.