Don't use deprecated class.

```
-import io.micrometer.core.instrument.util.StringUtils;
+import io.micrometer.common.util.StringUtils;
```

Author: tokuhirom

sample-manage-audience/src/main/java/com/linecorp/bot/messagingapidemoapp/controller/ManageAudienceController.java

@@ -42,7 +42,7 @@
 import com.linecorp.bot.model.manageaudience.request.CreateImpBasedAudienceGroupRequest;
 import com.linecorp.bot.model.manageaudience.request.UpdateAudienceGroupDescriptionRequest;
 
-import io.micrometer.core.instrument.util.StringUtils;
+import io.micrometer.common.util.StringUtils;
 import lombok.AllArgsConstructor;
 import lombok.extern.slf4j.Slf4j;
 

sample-manage-audience/src/main/java/com/linecorp/bot/messagingapidemoapp/controller/message/MessageHelper.java

@@ -28,7 +28,7 @@
 
 import com.linecorp.bot.model.message.Message;
 
-import io.micrometer.core.instrument.util.StringUtils;
+import io.micrometer.common.util.StringUtils;
 import lombok.AllArgsConstructor;
 
 @Component

sample-manage-audience/src/main/java/com/linecorp/bot/messagingapidemoapp/controller/message/MulticastController.java

@@ -36,7 +36,7 @@
 import com.linecorp.bot.model.Multicast;
 import com.linecorp.bot.model.message.Message;
 
-import io.micrometer.core.instrument.util.StringUtils;
+import io.micrometer.common.util.StringUtils;
 import lombok.AllArgsConstructor;
 import lombok.extern.slf4j.Slf4j;
 

sample-manage-audience/src/main/java/com/linecorp/bot/messagingapidemoapp/controller/message/NarrowcastController.java

@@ -56,7 +56,7 @@
 import com.linecorp.bot.model.narrowcast.filter.SubscriptionPeriodDemographicFilter.SubscriptionPeriod;
 import com.linecorp.bot.model.narrowcast.recipient.Recipient;
 
-import io.micrometer.core.instrument.util.StringUtils;
+import io.micrometer.common.util.StringUtils;
 import lombok.AllArgsConstructor;
 import lombok.extern.slf4j.Slf4j;