feat(deployments): cancel superseded deployments when a newer one is queued

app/Models/ApplicationDeploymentQueue.php

@@ -155,6 +155,7 @@ class ApplicationDeploymentQueue extends Model
         'horizon_job_id',
         'horizon_job_worker',
         'finished_at',
+        'deploy_started_at',
     ];
 
     /**
@@ -174,6 +175,7 @@ class ApplicationDeploymentQueue extends Model
     protected $casts = [
         'pull_request_id' => 'integer',
         'finished_at' => 'datetime',
+        'deploy_started_at' => 'datetime',
         'configuration_snapshot' => EncryptedArrayCast::class,
         'configuration_diff' => EncryptedArrayCast::class,
     ];