Attempt to make init-script compatible with Gradle 1.12

The `PluginManager` type wasn't introduced until Gradle 2.x.
Remove this type from the method signature in an attempt to allow this
file to be parsed with Gradle 1.12.

src/resources/init-scripts/gradle-build-action.inject-gradle-enterprise.init.gradle

@@ -168,7 +168,7 @@ if (GradleVersion.current() < GradleVersion.version('6.0')) {
     }
 }
 
-void applyPluginExternally(PluginManager pluginManager, String pluginClassName) {
+void applyPluginExternally(def pluginManager, String pluginClassName) {
     def externallyApplied = 'gradle.enterprise.externally-applied'
     def oldValue = System.getProperty(externallyApplied)
     System.setProperty(externallyApplied, 'true')