Use explicit classpath: prefix for Flyway location

Flyway implicitly adds classpath: to locations without a prefix but
resource loader fails to find migratons on the classpath without the
prefix.

Add an explicit classpath: prefix so that both Flyway and the resource
loader used to check the locations can find the migration location.

See gh-10807

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/flyway/FlywayProperties.java

@@ -42,7 +42,7 @@ public class FlywayProperties {
 	 * use vendor-specific locations.
 	 */
 	private List<String> locations = new ArrayList<>(
-			Collections.singletonList("db/migration"));
+			Collections.singletonList("classpath:db/migration"));
 
 	/**
 	 * Check that migration scripts location exists.