Stackdriver: Rename Stackdriver to Google Cloud Monitoring (#25807)

* Update backend

* Update frontend

* Keep old plugin id

* Update docs

* Place doc images to a new directory

* Legacy support for stackdriver-auto alignment

* Consistent plugin name

* Apply suggestions from code review

Co-authored-by: Marcus Efraimsson <marcus.efraimsson@gmail.com>
Co-authored-by: Tobias Skarhed <1438972+tskarhed@users.noreply.github.com>

* Update docs

* Update public/app/plugins/datasource/cloud-monitoring/README.md

Co-authored-by: Marcus Efraimsson <marcus.efraimsson@gmail.com>

* Add reference to the data source formerly being named Stackdriver

* Update pkg/models/datasource.go

Co-authored-by: Carl Bergquist <carl@grafana.com>

* Fix gofmt

Co-authored-by: Marcus Efraimsson <marcus.efraimsson@gmail.com>
Co-authored-by: Tobias Skarhed <1438972+tskarhed@users.noreply.github.com>
Co-authored-by: Carl Bergquist <carl@grafana.com>

docs/sources/menu.yaml

@@ -109,8 +109,8 @@
       name: Azure Monitor
     - link: /features/datasources/elasticsearch/
       name: Elasticsearch
-    - link: /features/datasources/stackdriver/
-      name: Google Stackdriver
+    - link: /features/datasources/cloudmonitoring/
+      name: Google Cloud Monitoring
     - link: /features/datasources/graphite/
       name: Graphite
     - link: /features/datasources/influxdb/