migrate flexmark-java 0_40_x to 0_42_0.xml

<?xml version="1.0" encoding="UTF-8"?>
<migrationMap>
    <name value="migrate flexmark-java 0.40.x to 0.42.0"/>
    <description value="Class migrations from flexmark-java 0.40.x to 0.42.0"/>
    <entry oldName="com.vladsch.flexmark.Extension" newName="com.vladsch.flexmark.util.builder.Extension" type="class"/>
</migrationMap>