Add Central Portal Snapshots repository for rundeck-core:6.0.0-SNAPSHOT

- OSSRH sunset on June 30, 2025, migrated to Central Portal
- Repository: https://central.sonatype.com/repository/maven-snapshots/
- Uses includeGroup('org.rundeck') to resolve all org.rundeck modules
- Resolves rundeck-core and its transitive dependencies
- Tested successfully in SSHJ plugin CI build
- Reference: https://central.sonatype.org/publish/publish-portal-upload/#consuming--snapshot-releases-for-your-project

Author: fdevans

build.gradle

@@ -28,6 +28,15 @@ project.version = '1.3.17-grails7-upgrade-test'
 
 repositories {
     mavenLocal()
+    maven {
+        name = 'Central Portal Snapshots'
+        url = 'https://central.sonatype.com/repository/maven-snapshots/'
+        
+        // Only search this repository for org.rundeck snapshots
+        content {
+            includeGroup('org.rundeck')
+        }
+    }
     mavenCentral()
 }