Since upgrading mariadb from 10.1 to 10.6 roundcube no longer connects to database when trying to log in. Examining httpd error log I find " Character set 'utf8' is not a compiled character set and is not specified in the '/usr/share/mysql/charsets/Index.xml' file" We were able circumvent the issue by adding the following section to /usr/share/mysql/charsets/Index.html.

<charset name="utf8">

  <family>Unicode</family>

  <description>UTF-8 Unicode</description>

  <alias>utf-8</alias>

  <collation name="utf8mb3_general_ci"  id="33">

   <flag>primary</flag>

   <flag>compiled</flag>

  </collation>

  <collation name="utf8mb3_bin"         id="83">

    <flag>binary</flag>

    <flag>compiled</flag>

  </collation>

</charset>

Note that this block is a copy of the "utf8mb3" block with only the charset name changed to "utf8".
This addition allowed roundcube to connect to the database and operate as expected.