Unlock plugins {...} syntax: publish plugin markers to Sonatype OSSRH (Maven Central)

Gradle plugins are usable with `plugins {...}` syntax if they have [plugin markers](https://docs.gradle.org/current/userguide/plugins.html#sec:plugin_markers)
Currently `app-gradle-plugin` does not publish one, so the users struggle.

It would be great if the markers were published.

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Currently, `app-gradle-plugin` uses [maven](https://docs.gradle.org/current/userguide/maven_plugin.html) to publish to Nexus. `maven` is deprecated for a long time, and it is probably time to migrate to [maven-publish](https://docs.gradle.org/current/userguide/publishing_maven.html)

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I don't know the way you publish/release, and I don't want to interrupt workflows, however, I guess the following might be a sane plan:

* Bump Gradle (e.g. to 6.8.2)
* Use `maven-publish` for publishing
* Use Gradle's `java-gradle-plugin` for plugin marker generation
* Use https://github.com/gradle-nexus/publish-plugin for Nexus staging. Just in case, staging enables you to create a "staging" repository, push multiple artifacts and release them at once (or drop the staged repo)
* Use Kotlin DSL for Gradle for less magic and better autocomplete

WDYT?