Restore conversion service binary compatibility

Change `ApplicationConversionService.getSharedInstance()` to again
return a `ConversionService` rather than `ApplicationConversionService`.

This restore binary compatibly with Spring Boot 2.0

Closes gh-14938

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/SpringApplication.java

@@ -59,6 +59,8 @@ import org.springframework.context.support.AbstractApplicationContext;
 import org.springframework.context.support.GenericApplicationContext;
 import org.springframework.core.GenericTypeResolver;
 import org.springframework.core.annotation.AnnotationAwareOrderComparator;
+import org.springframework.core.convert.ConversionService;
+import org.springframework.core.convert.support.ConfigurableConversionService;
 import org.springframework.core.env.CommandLinePropertySource;
 import org.springframework.core.env.CompositePropertySource;
 import org.springframework.core.env.ConfigurableEnvironment;
@@ -480,8 +482,10 @@ public class SpringApplication {
 	protected void configureEnvironment(ConfigurableEnvironment environment,
 			String[] args) {
 		if (this.addConversionService) {
+			ConversionService conversionService = ApplicationConversionService
+					.getSharedInstance();
 			environment.setConversionService(
-					ApplicationConversionService.getSharedInstance());
+					(ConfigurableConversionService) conversionService);
 		}
 		configurePropertySources(environment, args);
 		configureProfiles(environment, args);

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/convert/ApplicationConversionService.java

@@ -53,12 +53,16 @@ public class ApplicationConversionService extends FormattingConversionService {
 	}
 
 	/**
-	 * Return a shared default {@code ApplicationConversionService} instance, lazily
+	 * Return a shared default application {@code ConversionService} instance, lazily
 	 * building it once needed.
+	 * <p>
+	 * Note: This method actually returns an {@link ApplicationConversionService}
+	 * instance. However, the {@code ConversionService} signature has been preserved for
+	 * binary compatibility.
 	 * @return the shared {@code ApplicationConversionService} instance (never
 	 * {@code null})
 	 */
-	public static ApplicationConversionService getSharedInstance() {
+	public static ConversionService getSharedInstance() {
 		ApplicationConversionService sharedInstance = ApplicationConversionService.sharedInstance;
 		if (sharedInstance == null) {
 			synchronized (ApplicationConversionService.class) {