Remove usages of Gradle's jcenter() repository

Closes gh-10253

buildSrc/build.gradle

@@ -9,7 +9,6 @@ plugins {
 sourceCompatibility = 1.8
 
 repositories {
-	jcenter()
 	gradlePluginPortal()
 	mavenCentral()
 	maven { url 'https://repo.spring.io/plugins-release/' }

buildSrc/src/main/groovy/io/spring/gradle/convention/RepositoryConventionPlugin.groovy

@@ -35,11 +35,6 @@ class RepositoryConventionPlugin implements Plugin<Project> {
 				mavenLocal()
 			}
 			mavenCentral()
-			jcenter() {
-				content {
-					includeGroup "org.gretty"
-				}
-			}
 			if (isSnapshot) {
 				maven {
 					name = 'artifactory-snapshot'

buildSrc/src/test/java/io/spring/gradle/convention/RepositoryConventionPluginTests.java

@@ -107,7 +107,7 @@ public class RepositoryConventionPluginTests {
 		this.project.getPluginManager().apply(RepositoryConventionPlugin.class);
 
 		RepositoryHandler repositories = this.project.getRepositories();
-		assertThat(repositories).hasSize(5);
+		assertThat(repositories).hasSize(4);
 		assertThat((repositories.get(0)).getName()).isEqualTo("MavenLocal");
 	}
 
@@ -119,39 +119,33 @@ public class RepositoryConventionPluginTests {
 		this.project.getPluginManager().apply(RepositoryConventionPlugin.class);
 
 		RepositoryHandler repositories = this.project.getRepositories();
-		assertThat(repositories).hasSize(6);
+		assertThat(repositories).hasSize(5);
 		assertThat((repositories.get(0)).getName()).isEqualTo("MavenLocal");
 	}
 
 	private void assertSnapshotRepository(RepositoryHandler repositories) {
-		assertThat(repositories).extracting(ArtifactRepository::getName).hasSize(6);
+		assertThat(repositories).extracting(ArtifactRepository::getName).hasSize(5);
 		assertThat(((MavenArtifactRepository) repositories.get(0)).getUrl().toString())
 				.isEqualTo("https://repo.maven.apache.org/maven2/");
 		assertThat(((MavenArtifactRepository) repositories.get(1)).getUrl().toString())
-				.isEqualTo("https://jcenter.bintray.com/");
-		assertThat(((MavenArtifactRepository) repositories.get(2)).getUrl().toString())
 				.isEqualTo("https://repo.spring.io/snapshot/");
-		assertThat(((MavenArtifactRepository) repositories.get(3)).getUrl().toString())
+		assertThat(((MavenArtifactRepository) repositories.get(2)).getUrl().toString())
 				.isEqualTo("https://repo.spring.io/milestone/");
 	}
 
 	private void assertMilestoneRepository(RepositoryHandler repositories) {
-		assertThat(repositories).extracting(ArtifactRepository::getName).hasSize(5);
+		assertThat(repositories).extracting(ArtifactRepository::getName).hasSize(4);
 		assertThat(((MavenArtifactRepository) repositories.get(0)).getUrl().toString())
 				.isEqualTo("https://repo.maven.apache.org/maven2/");
 		assertThat(((MavenArtifactRepository) repositories.get(1)).getUrl().toString())
-				.isEqualTo("https://jcenter.bintray.com/");
-		assertThat(((MavenArtifactRepository) repositories.get(2)).getUrl().toString())
 				.isEqualTo("https://repo.spring.io/milestone/");
 	}
 
 	private void assertReleaseRepository(RepositoryHandler repositories) {
-		assertThat(repositories).extracting(ArtifactRepository::getName).hasSize(4);
+		assertThat(repositories).extracting(ArtifactRepository::getName).hasSize(3);
 		assertThat(((MavenArtifactRepository) repositories.get(0)).getUrl().toString())
 				.isEqualTo("https://repo.maven.apache.org/maven2/");
 		assertThat(((MavenArtifactRepository) repositories.get(1)).getUrl().toString())
-				.isEqualTo("https://jcenter.bintray.com/");
-		assertThat(((MavenArtifactRepository) repositories.get(2)).getUrl().toString())
 				.isEqualTo("https://repo.spring.io/release/");
 	}