Deprecate EnvironmentsCanaryIngressUpdate mutation

- Puts the mutation behind a FF
- Updates documentation to highlight the endpoint's deprecation

app/graphql/mutations/environments/canary_ingress/update.rb

@@ -5,6 +5,7 @@ module Mutations
     module CanaryIngress
       class Update < ::Mutations::BaseMutation
         graphql_name 'EnvironmentsCanaryIngressUpdate'
+        description '**Deprecated** This endpoint is planned to be removed along with certificate-based clusters. [See this epic](https://gitlab.com/groups/gitlab-org/configure/-/epics/8) for more information.'
 
         authorize :update_environment
 
@@ -18,7 +19,13 @@ module Mutations
                  required: true,
                  description: 'Weight of the Canary Ingress.'
 
+        REMOVAL_ERR_MSG = 'This endpoint was deactivated as part of the certificate-based' \
+          'kubernetes integration removal. See Epic:' \
+          'https://gitlab.com/groups/gitlab-org/configure/-/epics/8'
+
         def resolve(id:, **kwargs)
+          return { errors: [REMOVAL_ERR_MSG] } if cert_based_clusters_ff_disabled?
+
           environment = authorized_find!(id: id)
 
           result = ::Environments::CanaryIngress::UpdateService
@@ -33,6 +40,12 @@ module Mutations
           id = ::Types::GlobalIDType[::Environment].coerce_isolated_input(id)
           GitlabSchema.find_by_gid(id)
         end
+
+        private
+
+        def cert_based_clusters_ff_disabled?
+          Feature.disabled?(:certificate_based_clusters, default_enabled: :yaml, type: :ops)
+        end
       end
     end
   end

doc/api/graphql/reference/index.md

@@ -2372,6 +2372,8 @@ Input type: `EnableDevopsAdoptionNamespaceInput`
 
 ### `Mutation.environmentsCanaryIngressUpdate`
 
+**Deprecated** This endpoint is planned to be removed along with certificate-based clusters. [See this epic](https://gitlab.com/groups/gitlab-org/configure/-/epics/8) for more information.
+
 Input type: `EnvironmentsCanaryIngressUpdateInput`
 
 #### Arguments

spec/graphql/mutations/environments/canary_ingress/update_spec.rb

@@ -36,6 +36,20 @@ RSpec.describe Mutations::Environments::CanaryIngress::Update do
       it 'returns no errors' do
         expect(subject[:errors]).to be_empty
       end
+
+      context 'with certificate_based_clusters disabled' do
+        before do
+          stub_feature_flags(certificate_based_clusters: false)
+        end
+
+        it 'returns notice about feature removal' do
+          expect(subject[:errors]).to match_array([
+            'This endpoint was deactivated as part of the certificate-based' \
+            'kubernetes integration removal. See Epic:' \
+            'https://gitlab.com/groups/gitlab-org/configure/-/epics/8'
+          ])
+        end
+      end
     end
 
     context 'when service encounters a problem' do