Add UserHasBeenDeactivated plugin hook (#20894)

Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>

server/public/plugin/hooks.go

@@ -51,6 +51,7 @@ const (
 	deprecatedGetTopicMetadataByIdsID         = 33
 	ConfigurationWillBeSavedID                = 34
 	NotificationWillBePushedID                = 35
+	UserHasBeenDeactivatedID                  = 36
 	TotalHooksID                              = iota
 )
 
@@ -298,4 +299,9 @@ type Hooks interface {
 	//
 	// Minimum server version: 9.0
 	NotificationWillBePushed(pushNotification *model.PushNotification, userID string) (*model.PushNotification, string)
+
+	// UserHasBeenDeactivated is invoked when a user is deactivated.
+	//
+	// Minimum server version: 9.1
+	UserHasBeenDeactivated(c *Context, user *model.User)
 }