chore: update doc links from googleapis.dev to cloud.google.com (#643)

Co-authored-by: Anthonios Partheniou <partheniou@google.com>

.repo-metadata.json

@@ -2,7 +2,7 @@
     "name": "spanner",
     "name_pretty": "Cloud Spanner",
     "product_documentation": "https://cloud.google.com/spanner/docs/",
-    "client_documentation": "https://googleapis.dev/python/spanner/latest",
+    "client_documentation": "https://cloud.google.com/python/docs/reference/spanner/latest",
     "issue_tracker": "https://issuetracker.google.com/issues?q=componentid:190851%2B%20status:open",
     "release_level": "ga",
     "language": "python",

README.rst

@@ -22,7 +22,7 @@ workloads.
 .. |versions| image:: https://img.shields.io/pypi/pyversions/google-cloud-spanner.svg
    :target: https://pypi.org/project/google-cloud-spanner/
 .. _Cloud Spanner: https://cloud.google.com/spanner/
-.. _Client Library Documentation: https://googleapis.dev/python/spanner/latest
+.. _Client Library Documentation: https://cloud.google.com/python/docs/reference/spanner/latest
 .. _Product Documentation:  https://cloud.google.com/spanner/docs
 
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