Fix some Gradle deprecation warnings

A number of warnings still remain,
they seem a bit more complicated to fix.

build.gradle

@@ -503,8 +503,8 @@ subprojects {
     publishing {
         publications {
             POI(MavenPublication) {
-                groupId 'org.apache.poi'
-                artifactId project.archivesBaseName
+                groupId = 'org.apache.poi'
+                artifactId = project.archivesBaseName
 
                 from components.java
 

poi-integration/build.gradle

@@ -156,9 +156,13 @@ test {
     }
 }
 
-javadoc { enabled(false) }
+javadoc {
+    enabled = false
+}
 
-sourcesJar { enabled(false) }
+sourcesJar {
+    enabled = false
+}
 
 generateMetadataFileForPOIPublication.enabled = false
 publishPOIPublicationToMavenLocal.enabled = false