Part of https://github.com/elastic/kibana/issues/154715

The `.fleet-secrets` system index should just have a value key. 

Kibana system should only have write and delete access NOT read access.

Fleet server will have read access only. 

Example document:

```
// example document from the .fleet-secrets index 
{
	_id: 'abcd1234'
	_source: {
	   value: "abcd12345678",
      }
}
```
 
x-pack/plugin/fleet/src/main/java/org/elasticsearch/xpack/fleet/Fleet.java
.setPrimaryIndex(".fleet-artifacts-" + CURRENT_INDEX_VERSION)
            .setIndexPattern(".fleet-artifacts*")
            .setAliasName(".fleet-artifacts")
            .setDescription("Fleet artifacts")
            .build();
    }
    private SystemDataStreamDescriptor fleetActionsResultsDescriptor() {
        final String source = loadTemplateSource("/fleet-actions-results.json");
        try (XContentParser parser = XContentType.JSON.xContent().createParser(XContentParserConfiguration.EMPTY, source)) {
            ComposableIndexTemplate composableIndexTemplate = ComposableIndexTemplate.parse(parser);
            return new SystemDataStreamDescriptor(
                ".fleet-actions-results",
                "Result history of fleet actions",
                SystemDataStreamDescriptor.Type.EXTERNAL,
                composableIndexTemplate,
                Map.of(),
                ALLOWED_PRODUCTS,
                ExecutorNames.DEFAULT_SYSTEM_DATA_STREAM_THREAD_POOLS
            );
        } catch (IOException e) {
            throw new UncheckedIOException(e);
        }
    }
    @Override
    public void cleanUpFeature(ClusterService clusterService, Client client, ActionListener<ResetFeatureStateStatus> listener) {
        Collection<SystemDataStreamDescriptor> dataStreamDescriptors = getSystemDataStreamDescriptors();
        if (dataStreamDescriptors.isEmpty() == false) {
            try {
                Request request = new Request(
                    dataStreamDescriptors.stream()