App Engine PHP sample uses deprecated runtime (php81)

The App Engine Standard PHP sample currently specifies `runtime: php81` in `app.yaml`.

As of now, `php81` is end of support and no longer accepted by App Engine Standard, which causes deployments to fail with the following error:

ERROR: (gcloud.app.deploy) INVALID_ARGUMENT: Error(s) encountered validating runtime. Runtime php81 is end of support and no longer allowed.

To avoid deployment failures for users, especially those following tutorials or labs, I propose updating the runtime to a supported version (e.g. `php83`, or `php82` if preferred).

Proposed change:
- Update appengine/standard/helloworld/app.yaml` from `runtime: php81` to a currently supported PHP runtime.

I’m happy to submit a pull request if this change is acceptable.