Switch to jakarta.xml.ws.handler.soap.SOAPHandler and include jakarta.xml.ws-api in your dependencies:
<dependency>
<groupId>jakarta.xml.ws</groupId>
<artifactId>jakarta.xml.ws-api</artifactId>
<version>4.0.0</version>
</dependency>
If you are still on versions that use javax instead of jakarta, include jaxws-api in your dependencies:
<dependency>
<groupId>javax.xml.ws</groupId>
<artifactId>jaxws-api</artifactId>
<version>2.3.1</version>
</dependency>
Jakarta EE 9, which moved the API namespace from javax to jakarta, was released on December 8, 2020 which happens to fall between the releases of Java 15 and Java 16, but it will really depend more on your other dependencies than the version of Java you’re using.