Move plugin declarations to plugin syntax

Note: this syntax automatically applies the plugin to the root project.
Adding `apply false` in the plugin declaration disables that.

Issue: SPR-15885

gradle/docs.gradle

@@ -14,8 +14,6 @@
  *  limitations under the License.
  */
 
-apply plugin: "org.asciidoctor.convert"
-
 asciidoctor {
 	sources {
 		include '*.adoc'