fix(python): specify release level of iamconnectorcredentials (#5216)

iamconnectorcredentials/v1alpha should only be generated for python, and
should have a preview release level.

Fixes #5214

Author: jskeet

internal/serviceconfig/sdk.yaml

@@ -1698,6 +1698,11 @@
   release_level:
     go: beta
     java: preview
+- path: google/cloud/iamconnectorcredentials/v1alpha
+  languages:
+    - python
+  release_level:
+    all: preview
 - path: google/cloud/iap/v1
   documentation_uri: https://cloud.google.com/iap
   languages: