Reuse optional configuration name constant

See gh-19620

buildSrc/src/main/java/org/springframework/boot/build/optional/OptionalDependenciesPlugin.java

@@ -44,7 +44,7 @@ public class OptionalDependenciesPlugin implements Plugin<Project> {
 
 	@Override
 	public void apply(Project project) {
-		Configuration optional = project.getConfigurations().create("optional");
+		Configuration optional = project.getConfigurations().create(OPTIONAL_CONFIGURATION_NAME);
 		optional.attributes((attributes) -> attributes.attribute(Usage.USAGE_ATTRIBUTE,
 				project.getObjects().named(Usage.class, Usage.JAVA_RUNTIME)));
 		project.getPlugins().withType(JavaPlugin.class, (javaPlugin) -> {

buildSrc/src/test/java/org/springframework/boot/build/optional/OptionalDependenciesPluginIntegrationTests.java

@@ -57,7 +57,7 @@ public class OptionalDependenciesPluginIntegrationTests {
 			out.println("}");
 		}
 		BuildResult buildResult = runGradle("printConfigurations");
-		assertThat(buildResult.getOutput()).contains("optional");
+		assertThat(buildResult.getOutput()).contains(OptionalDependenciesPlugin.OPTIONAL_CONFIGURATION_NAME);
 	}
 
 	@Test