fix(appsync): ttl property of CachingConfig is not required (#17981)

According to the current CloudFormaton spec for [AWS::AppSync::Resolver CachingConfig][1], the `Ttl` property is not required. But if it's not provided, the deploy will fail with the message:

>  TTL value cannot be smaller than 1 second and larger than 3600 seconds. (Service: AWSAppSync; Status Code: 400; Error Code: BadRequestException

The AppSync team has confirmed that the property is indeed required and they will push a change to the spec. This change is proactively making the property required on the CDK.

Fixes #17925.

BREAKING CHANGE: The `CachingConfig#ttl` property is now required. 

[1]: https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-appsync-resolver-cachingconfig.html#cfn-appsync-resolver-cachingconfig-ttl
----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-appsync/lib/caching-config.ts

@@ -16,7 +16,6 @@ export interface CachingConfig {
    * The TTL in seconds for a resolver that has caching enabled.
    * Valid values are between 1 and 3600 seconds.
    *
-   * @default - No TTL
    */
-  readonly ttl?: Duration;
+  readonly ttl: Duration;
 }