LUCENE-10138: Use maven central to resolve third-party gradle plugins (#336)

The gradle plugin portal uses jcenter to resolve third-party plugins, which
can be flaky. This commit instructs gradle to look first in maven central,
and only use the plugin portal for gradle's own plugins.

settings.gradle

@@ -15,6 +15,13 @@
  * limitations under the License.
  */
 
+pluginManagement {
+    repositories {
+        mavenCentral()
+        gradlePluginPortal()
+    }
+}
+
 rootProject.name = "lucene-root"
 
 includeBuild("dev-tools/missing-doclet")