Gradle - could not find method runtime(), but I don't have it in my project

I'm new to gradle, and having some difficulties when migrating project from gradle 6.8.1 to 7.0.2. Here is mine build.gradle:

buildscript {
    dependencies {
        classpath "com.generali.gef.gradle:gef-ws-xsd-starter-plugin:$GEF_VERSION"
    }
}

apply plugin: 'java-library'
apply plugin: 'gef-ws-xsd-starter-plugin'

dependencies {
    implementation 'com.generali.gef:gef-ws-xsd',
            'gef-ws-xsd-starter-plugin'
}

In gradle 6.8.1, it all worked just fine. However, when I changed the version to 7.0.2, this script started throwing this error:

A problem occurred evaluating project ':demo-ws-xsd'.
> Failed to apply plugin 'gef-ws-xsd-starter-plugin'.
   > Could not find method runtime() for arguments [org.glassfish.jaxb:jaxb-runtime] on object of type org.gradle.api.internal.artifacts.dsl.dependencies.DefaultDependencyHandler.

I know that runtime keyword was removed in gradle 7, however this keyword is not anywhere in my .gradle files. Any ideas what could be wrong?

edit

it is thrown by line 8 of the file, meaning

apply plugin: 'gef-ws-xsd-starter-plugin'