[#4209] chore replace hardcoded value with std definition (#6354)

### What changes were proposed in this pull request?
Use standard definition lib value to replace the hardcoded value: UTF-8.
### Why are the changes needed?

Fix: #4209

### Does this PR introduce _any_ user-facing change?
no

### How was this patch tested?
no

server-common/src/main/java/org/apache/gravitino/server/authentication/KerberosServerUtils.java

@@ -16,12 +16,11 @@
 
 import java.io.File;
 import java.io.IOException;
-import java.io.UnsupportedEncodingException;
 import java.lang.reflect.InvocationTargetException;
 import java.net.InetAddress;
 import java.net.UnknownHostException;
 import java.nio.ByteBuffer;
-import java.nio.charset.IllegalCharsetNameException;
+import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;
 import java.util.HashSet;
 import java.util.Iterator;
@@ -353,11 +352,8 @@ DER get(int... tags) {
     }
 
     String getAsString() {
-      try {
-        return new String(bb.array(), bb.arrayOffset() + bb.position(), bb.remaining(), "UTF-8");
-      } catch (UnsupportedEncodingException e) {
-        throw new IllegalCharsetNameException("UTF-8"); // won't happen.
-      }
+      return new String(
+          bb.array(), bb.arrayOffset() + bb.position(), bb.remaining(), StandardCharsets.UTF_8);
     }
 
     @Override