Use ZoneOffset.UTC instead of ZoneId.of("Z") (#353)

## What is the goal of this PR?

ZoneId.of("Z") is forbidden in Java 16, so we've replaced it with ZoneOffset.UTC.

## What are the changes implemented in this PR?

Use ZoneOffset.UTC instead of ZoneId.of("Z")

common/rpc/RequestBuilder.java

@@ -38,6 +38,7 @@
 
 import java.time.LocalDateTime;
 import java.time.ZoneId;
+import java.time.ZoneOffset;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
@@ -721,7 +722,7 @@ public static ConceptProto.Attribute.Value protoStringAttributeValue(String valu
             }
 
             public static ConceptProto.Attribute.Value protoDateTimeAttributeValue(LocalDateTime value) {
-                long epochMillis = value.atZone(ZoneId.of("Z")).toInstant().toEpochMilli();
+                long epochMillis = value.atZone(ZoneOffset.UTC).toInstant().toEpochMilli();
                 return ConceptProto.Attribute.Value.newBuilder().setDateTime(epochMillis).build();
             }
         }

concept/thing/AttributeImpl.java

@@ -31,6 +31,7 @@
 import java.time.Instant;
 import java.time.LocalDateTime;
 import java.time.ZoneId;
+import java.time.ZoneOffset;
 import java.util.stream.Stream;
 
 import static com.vaticle.typedb.client.common.exception.ErrorMessage.Concept.BAD_VALUE_TYPE;
@@ -488,7 +489,7 @@ public AttributeImpl.DateTime.Remote asRemote(TypeDBTransaction transaction) {
         }
 
         private static LocalDateTime toLocalDateTime(long rpcDatetime) {
-            return LocalDateTime.ofInstant(Instant.ofEpochMilli(rpcDatetime), ZoneId.of("Z"));
+            return LocalDateTime.ofInstant(Instant.ofEpochMilli(rpcDatetime), ZoneOffset.UTC);
         }
 
         public static class Remote extends AttributeImpl.Remote<LocalDateTime> implements Attribute.DateTime.Remote {