Fix Rubocop complains

Fixes:

1. You specified required: true, in Rails > 5.0 the required option
   is deprecated and you want to use optional: false.
   In most configurations, this is the default
   and you can omit this option altogether

2. Enum defined as an array found in operator enum declaration.
   Use hash syntax instead.

app/models/prometheus_alert.rb

@@ -9,9 +9,9 @@ class PrometheusAlert < ApplicationRecord
     gt: ">"
   }.freeze
 
-  belongs_to :environment, required: true, validate: true, inverse_of: :prometheus_alerts
-  belongs_to :project, required: true, validate: true, inverse_of: :prometheus_alerts
-  belongs_to :prometheus_metric, required: true, validate: true, inverse_of: :prometheus_alerts
+  belongs_to :environment, optional: false, validate: true, inverse_of: :prometheus_alerts
+  belongs_to :project, optional: false, validate: true, inverse_of: :prometheus_alerts
+  belongs_to :prometheus_metric, optional: false, validate: true, inverse_of: :prometheus_alerts
 
   has_many :prometheus_alert_events, inverse_of: :prometheus_alert
   has_many :related_issues, through: :prometheus_alert_events
@@ -22,7 +22,7 @@ class PrometheusAlert < ApplicationRecord
   validate :require_valid_environment_project!
   validate :require_valid_metric_project!
 
-  enum operator: [:lt, :eq, :gt]
+  enum operator: { lt: 0, eq: 1, gt: 2 }
 
   delegate :title, :query, to: :prometheus_metric