Assignee and @mention user searches using the last name don't work when the 'last_name' format doesn't match the 'display name'

Issue Summary

When trying to make an '@' mention, or select an assignee on your issues, you may not get the proper results when searching for the last name.
Other user-picker custom or system fields should not be affected.

This problem is originated due to a performance improvement change made to how user searches work in Jira starting on version 8.19.1.

In the past, Jira would try to match the search prefix with every word of the display name (eg. "John" & "Doe"), however, now we match the prefix (using startsWith) only with the whole display name ("John Doe"). While the search is also applied to the 'first_name' and 'last_name' data, in case the 'last_name' information doesn't match the same format as the display name (eg. using a different set of characters such as "Джон" & "Доу", which translates to "John Doe"), Jira won't be able to find results.

  
Note:  While this report is very similar to JSWSERVER-21109 (which affects 'third' name searches), the causes for each are different. 

Steps to Reproduce

1. Create a new user in Jira with the following fields.
   The Jira UI will display the user name with the standard western character set, however on the backend, the individual names will be set using a different set.
   • first_name = 'Джон'
   • lower_first_name = 'джон'
   • last_name = 'Доу'
   • lower_last_name = 'доу'
   • display_name='John Doe'
   • lower_display_name = 'john doe'
2. Try to create an '@' mention, or select an assignee searching for its last name - 'Doe'.
   • If you initially search for 'John' on user mentions, you may get temporary results due to Jira loading the values on its front-end cache, however, this will be invalidated when refreshing the page.

Expected Results

Since 'Doe' is set on the display name, we should get results when searching for it.

Actual Results

Since 'Doe' is not the first name of the display name, and the individual last_name is set as 'Доу', no results will be returned.

Workaround

• Search using the user's first name.
• In case the user originates from an external LDAP connection, adjust the LDAP parameters to fetch the first and last names in the same format (character set) as the display name.