[sdk-platform-java] ObsoleteApi: `getDefualtEndpoint()` in the generated ServiceStubSettings

Changes:
- The `getDefaultEndpoint()` will be marked with `@ObsoleteApi` annotation with the intention of removing it in a future major version bump.
- The generated ServiceStubSetting classes will no longer be passing the a default endpoint via the `getDefaultEndpoint()` method.

Example of the generated endpoints:
- [AccessApproval](https://github.com/googleapis/google-cloud-java/blob/e4107b2bc4f018119ae62c1af63484a52a1d37e1/java-accessapproval/google-cloud-accessapproval/src/main/java/com/google/cloud/accessapproval/v1/stub/AccessApprovalStubSettings.java#L281-L283)
- [Redis](https://github.com/googleapis/google-cloud-java/blob/e4107b2bc4f018119ae62c1af63484a52a1d37e1/java-redis/google-cloud-redis/src/main/java/com/google/cloud/redis/v1/stub/CloudRedisStubSettings.java#L404-L406)

## What's Changing (Behavior/ Code)
There are no changes required on the user's end. The client will automatically determine the endpoint and the default value used remains `{service}.googleapis.com`. If there are user configurations for the endpoint, the configured value will be used.

## How to set a custom endpoint
See [this](https://cloud.google.com/java/docs/setup#configure_endpoints_for_the_client_library) as an example of how to configure the endpoint