In the document https://confluence.atlassian.com/jirasoftware/jira-software-8-13-x-upgrade-notes-1018783378.html, in the section Known issue: Jira doesn’t recognize a MySQL database after the upgrade, where it states

If you're using MySQL 5.6, you will need to upgrade your database first, as this version is no longer supported.

Please add a note saying that the utf8_bin collation/utf8 charset has to be converted to utf8mb4_bin/utf8mb4. there's a KB with the steps at https://confluence.atlassian.com/kb/how-to-fix-the-collation-and-character-set-of-a-mysql-database-manually-744326173.html