Update some samples for PHP Flex to use newer runtimes

Author: kennethye1

appengine/flexible/datastore/app.yaml

@@ -3,3 +3,5 @@ env: flex
 
 runtime_config:
   document_root: .
+  operating_system: ubuntu22
+  runtime_version: 8.3

appengine/flexible/helloworld/app.yaml

@@ -3,6 +3,8 @@ env: flex
 
 runtime_config:
   document_root: web
+  operating_system: ubuntu22
+  runtime_version: 8.3
 
 # This sample incurs costs to run on the App Engine flexible environment.
 # The settings below are to reduce costs during testing and are not appropriate

appengine/flexible/storage/app.yaml

@@ -3,6 +3,8 @@ env: flex
 
 runtime_config:
   document_root: .
+  operating_system: ubuntu22
+  runtime_version: 8.3
 
 # [START gae_flex_storage_yaml]
 env_variables: