chore: update Nexus publishing to use Central Portal API

Configure sonatype repository to use the new Central Portal staging API endpoints as recommended by the migration guide.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: nakamura-to

build.gradle.kts

@@ -311,7 +311,11 @@ rootProject.apply {
 
     nexusPublishing {
         repositories {
-            sonatype()
+            // see https://central.sonatype.org/publish/publish-portal-ossrh-staging-api/#configuration
+            sonatype {
+                nexusUrl.set(uri("https://ossrh-staging-api.central.sonatype.com/service/local/"))
+                snapshotRepositoryUrl.set(uri("https://central.sonatype.com/repository/maven-snapshots/"))
+            }
         }
         packageGroup.set("org.seasar")
     }