Remove repo.spring.io/plugins-release from Gradle

The https://repo.spring.io/plugins-release requires an authentication now.
Replace it with a `mavenCentral()`

build.gradle

@@ -3,7 +3,7 @@ buildscript {
     ext.isCI = System.getenv('GITHUB_ACTION') || System.getenv('bamboo_buildKey')
     repositories {
         gradlePluginPortal()
-        maven { url 'https://repo.spring.io/plugins-release' }
+        mavenCentral()
         if (version.endsWith('SNAPSHOT')) {
             maven { url 'https://repo.spring.io/snapshot' }
         }