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
.setAliasName(".fleet-policies-leader")
            .setDescription("Fleet Policies leader")
            .build();
    }
    private SystemIndexDescriptor fleetServersSystemIndexDescriptors() {
        PutIndexTemplateRequest request = new PutIndexTemplateRequest();
        request.source(loadTemplateSource("/fleet-servers.json"), XContentType.JSON);
        return SystemIndexDescriptor.builder()
            .setType(Type.EXTERNAL_MANAGED)
            .setAllowedElasticProductOrigins(ALLOWED_PRODUCTS)
            .setOrigin(FLEET_ORIGIN)
            .setVersionMetaKey(VERSION_KEY)
            .setMappings(request.mappings())
            .setSettings(request.settings())
            .setPrimaryIndex(".fleet-servers-" + CURRENT_INDEX_VERSION)
            .setIndexPattern(".fleet-servers*")
            .setAliasName(".fleet-servers")
            .setDescription("Fleet servers")
            .build();
    }
    private SystemIndexDescriptor fleetArtifactsSystemIndexDescriptors() {
        PutIndexTemplateRequest request = new PutIndexTemplateRequest();
        request.source(loadTemplateSource("/fleet-artifacts.json"), XContentType.JSON);
        return SystemIndexDescriptor.builder()
            .setType(Type.EXTERNAL_MANAGED)
            .setAllowedElasticProductOrigins(ALLOWED_PRODUCTS)
            .setOrigin(FLEET_ORIGIN)
            .setVersionMetaKey(VERSION_KEY)
            .setMappings(request.mappings())
            .setSettings(request.settings())