Update config_server_interface.py (#467)

## Issue
charm uses incorrect config `Config.K8S_SUBSTRATE `

## Solution
make it use the correct one

lib/charms/mongodb/v0/config_server_interface.py

@@ -42,7 +42,7 @@
 
 # Increment this PATCH version before using `charmcraft publish-lib` or reset
 # to 0 if you are raising the major API version
-LIBPATCH = 10
+LIBPATCH = 11
 
 
 class ClusterProvider(Object):
@@ -373,7 +373,7 @@ def is_mongos_running(self) -> bool:
         connection_uri = f"mongodb://{self.charm.get_mongos_host()}"
 
         # use the mongos port for k8s charms and external connections on VM
-        if self.substrate == Config.K8S_SUBSTRATE or self.charm.is_external_client:
+        if self.substrate == Config.Substrate.K8S or self.charm.is_external_client:
             connection_uri = connection_uri + f":{Config.MONGOS_PORT}"
 
         with MongosConnection(None, connection_uri) as mongo: