data-source/aws_availability_zones: Fix group_names attribute to be Computed instead of Optional (#14412)

Reference: #14405

Output from acceptance testing:

```
--- PASS: TestAccAWSAvailabilityZones_Filter (7.67s)
--- PASS: TestAccAWSAvailabilityZones_basic (7.83s)
--- PASS: TestAccAWSAvailabilityZones_AllAvailabilityZones (7.88s)
--- PASS: TestAccAWSAvailabilityZones_stateFilter (7.88s)
--- PASS: TestAccAWSAvailabilityZones_ExcludeNames (9.35s)
--- PASS: TestAccAWSAvailabilityZones_ExcludeZoneIds (9.41s)
```

Author: bflad

aws/data_source_aws_availability_zones.go

@@ -34,7 +34,7 @@ func dataSourceAwsAvailabilityZones() *schema.Resource {
 			"filter": ec2CustomFiltersSchema(),
 			"group_names": {
 				Type:     schema.TypeSet,
-				Optional: true,
+				Computed: true,
 				Elem:     &schema.Schema{Type: schema.TypeString},
 			},
 			"names": {