Add deprecation notice to NetworkPolicyConnection

Changelog: deprecated
EE: true

doc/api/graphql/reference/index.md

@@ -14416,6 +14416,10 @@ four standard [pagination arguments](#connection-pagination-arguments):
 
 Network Policies of the project.
 
+WARNING:
+**Deprecated** in 14.8.
+Network policies are deprecated and will be removed in GitLab 15.0.
+
 Returns [`NetworkPolicyConnection`](#networkpolicyconnection).
 
 This field returns a [connection](#connections). It accepts the

ee/app/graphql/ee/types/project_type.rb

@@ -199,6 +199,7 @@ module EE
         field :network_policies,
               ::Types::NetworkPolicyType.connection_type,
               null: true,
+              deprecated: { reason: 'Network policies are deprecated and will be removed in GitLab 15.0', milestone: '14.8' },
               description: 'Network Policies of the project',
               resolver: ::Resolvers::NetworkPolicyResolver