Update the XML scheme references in your XML descriptor files. Check the Jakarta EE 9 schemas. In most cases, it is a simple replace of http://xmlns.jcp.org/xml/ns/ with https://jakarta.ee/xml/ns/ in addition to the schema version number.
Before

<persistence version="2.2" xmlns="http://xmlns.jcp.org/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence http://xmlns.jcp.org/xml/ns/persistence/persistence_2_2.xsd">

After

<persistence version="3.0" xmlns="https://jakarta.ee/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="https://jakarta.ee/xml/ns/persistence https://jakarta.ee/xml/ns/persistence/persistence_3_0.xsd">

Reference complete-duke