Add a marker artifact for the io.spring.javaformat gradle plugin

[larsgrefer]

see https://docs.gradle.org/7.0/userguide/plugins.html#sec:plugin_markers
see #309

This should re-enable the usage of Gradles "new" plugins DSL

plugins {
    id 'io.spring.javaformat' version '0.0.32'
}

as long as mavenCentral() is added as plugin repository in the settings.gradle:

pluginManagement {
    repositories {
        gradlePluginPortal()
        mavenCentral()
    }
}