Use Saved Objects API for exporting and importing dashboards (#27220)

This PR adopts the Saved Objects API for exporting and importing dashboards and index templates. We have agreed to provide essential support, so there are a few limitations:

* You can only import assets they have exported using Beats 7.15 and the assets we ship with Beats.
* You can only export new assets, not legacy format.

We have to move forward to let Kibana remove the old API.

Try to setup the dashboards of a Beat:

```sh
./filebeat setup --dashboards
```

Export dashboards:

```sh
./filebeat export dashboard --id {my-id}
```
and:

```sh
go run dev-tools/cmd/dashboards/export_dashboards.go -dashboard {my-id}
```

The changeset is huge, but most of it comes from converting the dashboards. I suggest you use the file filter and disable json, ndjson and yml files. The changes are moving json to ndjson format and changing the extensions in the module.yml files from json to ndjson.

Please also test it manually with both the setup command and `export_dashboards.go`.

Closes #20672

CHANGELOG.next.asciidoc

@@ -20,6 +20,13 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Remove the deprecated `xpack.monitoring.*` settings. Going forward only `monitoring.*` settings may be used. {issue}9424[9424] {pull}18608[18608]
 - Add daemonset.name in pods controlled by DaemonSets {pull}26808[26808], {issue}25816[25816]
 - Kubernetes autodiscover fails in node scope if node name cannot be discovered {pull}26947[26947]
+- Use alias to report container image in k8s metadata. {pull}24380[24380]
+- Set `cleanup_timeout` to zero by default in docker and kubernetes autodiscover in all beats except Filebeat where it is kept to 60 seconds. {pull}24681[24681]
+- Update to ECS 1.9.0. {pull}24909[24909]
+- Remove the deprecated `xpack.monitoring.*` settings. Going forward only `monitoring.*` settings may be used. {issue}9424[9424] {pull}18608[18608]
+- Add daemonset.name in pods controlled by DaemonSets {pull}26808[26808], {issue}25816[25816]
+- Kubernetes autodiscover fails in node scope if node name cannot be discovered {pull}26947[26947]
+- Loading Kibana assets (dashboards, index templates) rely on Saved Object API. So to provide a reliable service, Beats can only import and export dasbhboards using at least Kibana 7.15. {issue}20672[20672] {pull}27220[27220]
 
 *Auditbeat*