Add plugins.gradle.org repo to build script

To avoid the `401 from server: Unauthorized`
when we try to reach `https://repo.spring.io/plugins-release`
it is better to point to some more central repository with
no authentication requirements.
Most of the Gradle plugins are managed in the `https://plugins.gradle.org/m2`
repo, not Maven Central

build.gradle

@@ -2,6 +2,7 @@ buildscript {
 	ext.kotlinVersion = '1.4.20'
 	repositories {
 		mavenCentral()
+		maven { url 'https://plugins.gradle.org/m2' }
 		maven { url 'https://repo.spring.io/plugins-release' }
 	}
 	dependencies {