Reinstate Maven Central as a source for dependency upgrades

See gh-42333

buildSrc/src/main/java/org/springframework/boot/build/bom/bomr/UpgradeBom.java

@@ -43,6 +43,7 @@ public abstract class UpgradeBom extends UpgradeDependencies {
 	}
 
 	private void addOpenSourceRepositories() {
+		getRepositoryNames().add(ArtifactRepositoryContainer.DEFAULT_MAVEN_CENTRAL_REPO_NAME);
 		getProject().getRepositories().withType(MavenArtifactRepository.class, (repository) -> {
 			String name = repository.getName();
 			if (name.startsWith("spring-") && !name.endsWith("-snapshot")) {