resource/aws_apigatewayv2_model: Replace deprecated validation.Valida…

…teJsonString with validation.StringIsJSON

Reference: #11844
Reference: #12087

Output from acceptance testing:

```
--- PASS: TestAccAWSAPIGatewayV2Model_disappears (18.58s)
--- PASS: TestAccAWSAPIGatewayV2Model_basic (20.32s)
--- PASS: TestAccAWSAPIGatewayV2Model_AllAttributes (46.23s)
```

aws/resource_aws_apigatewayv2_model.go

@@ -52,7 +52,7 @@ func resourceAwsApiGatewayV2Model() *schema.Resource {
 				Required: true,
 				ValidateFunc: validation.All(
 					validation.StringLenBetween(0, 32768),
-					validation.ValidateJsonString,
+					validation.StringIsJSON,
 				),
 				DiffSuppressFunc: suppressEquivalentJsonDiffs,
 				StateFunc: func(v interface{}) string {