Move some more atomic values under Channels (#18837)

* Move some more atomic values under Channels

The following fields were moved:

```
asymmetricSigningKey atomic.Value
clientConfig         atomic.Value
clientConfigHash     atomic.Value
limitedClientConfig  atomic.Value
```

And also moved the initialization order from NewServer
to Channels.Start to better reflect the order of things.

Removed AsymmetricSigningKey from ConfigService
as it was no longer used.

Removed calling regenerateClientConfig during startup explicitly
because it was anyways called from ensureAsymmetricSigningKey.

https://community-daily.mattermost.com/boards/workspace/zyoahc9uapdn3xdptac6jb69ic/285b80a3-257d-41f6-8cf4-ed80ca9d92e5/495cdb4d-c13a-4992-8eb9-80cfee2819a4?c=87df1e15-588e-49ff-8bd1-ffa9651b8c82
```release-note
NONE
```

* Fix lint error

```release-note
NONE
```

services/configservice/configservice.go

@@ -4,8 +4,6 @@
 package configservice
 
 import (
-	"crypto/ecdsa"
-
 	"github.com/mattermost/mattermost-server/v6/model"
 )
 
@@ -14,5 +12,4 @@ type ConfigService interface {
 	Config() *model.Config
 	AddConfigListener(func(old, current *model.Config)) string
 	RemoveConfigListener(string)
-	AsymmetricSigningKey() *ecdsa.PrivateKey
 }