chore: remove "Signing publications" from the logging output

The message was not adding much, and it was emitted during configuration phase,
so it was not even aligned with the actual signing work.

sigstore-gradle/sigstore-gradle-sign-plugin/src/main/kotlin/dev.sigstore.sign.gradle.kts

@@ -14,16 +14,13 @@
  * limitations under the License.
  *
  */
-import dev.sigstore.sign.SigstoreSignExtension
-
 plugins {
     id("dev.sigstore.sign-base")
     id("maven-publish")
 }
 
 plugins.withId("publishing") {
-    logger.lifecycle("Signing publications")
-    configure<SigstoreSignExtension> {
-        sign(publications = the<PublishingExtension>().publications)
+    sigstoreSign {
+        sign(publications = publishing.publications)
     }
 }