Migrate the removed maven plugin to maven-publish

Gradle 7.0 removed the legacy maven plugin together with the uploadArchives task, mavenDeployer, MavenDeployment and the install task, so a build that still applies it fails before any task runs. In a Groovy build script whose uploadArchives task deploys through mavenDeployer to a release repository and an optional snapshot repository, with an optional pom.project { } and signing.signPom(..), the plugin is switched to maven-publish and uploadArchives becomes a publishing { } block. Its publication holds the Java component and the other artifacts of the archives configuration, and signing signs the publications instead of the archives configuration. Where the plugin is applied but nothing of it is used, it is removed. A build that uses the plugin in any other way is left unchanged, so that it keeps Gradle's error message. See the Gradle upgrade guide.