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/features/datasources/_index.md

@@ -24,7 +24,7 @@ The following data sources are officially supported:
 * [AWS CloudWatch]({{< relref "cloudwatch.md" >}})
 * [Azure Monitor]({{< relref "azuremonitor.md" >}})
 * [Elasticsearch]({{< relref "elasticsearch.md" >}})
-* [Google Stackdriver]({{< relref "stackdriver.md" >}})
+* [Google Cloud Monitoring]({{< relref "cloudmonitoring.md" >}})
 * [Graphite]({{< relref "graphite.md" >}})
 * [InfluxDB]({{< relref "influxdb.md" >}})
 * [Loki]({{< relref "loki.md" >}})