Deprecate deleting iterations via the GraphQL API

Manual iteration cadences will be removed in the future
and only automatic iteration cadecnes will be allowed.
Automatic cadences will manage iterations automatically.

Changelog: deprecated
EE: true

doc/api/graphql/reference/index.md

@@ -3234,6 +3234,10 @@ Input type: `iterationCreateInput`
 
 ### `Mutation.iterationDelete`
 
+WARNING:
+**Deprecated** in 14.10.
+Manual iteration management is deprecated. Only automatic iteration cadences will be supported in the future.
+
 Input type: `IterationDeleteInput`
 
 #### Arguments

ee/app/graphql/ee/types/deprecated_mutations.rb

@@ -5,7 +5,11 @@ module EE
     module DeprecatedMutations
       extend ActiveSupport::Concern
 
+      ITERATION_DEPRECATION_MESSAGE = 'Manual iteration management is deprecated. Only automatic iteration cadences will be supported in the future'
+
       prepended do
+        mount_mutation ::Mutations::Iterations::Delete,
+          deprecated: { reason: ITERATION_DEPRECATION_MESSAGE, milestone: '14.10' }
         mount_aliased_mutation 'CreateIteration', ::Mutations::Iterations::Create,
           deprecated: { reason: 'Use iterationCreate', milestone: '14.0' }
       end

ee/app/graphql/ee/types/mutation_type.rb

@@ -23,7 +23,6 @@ module EE
         mount_mutation ::Mutations::Projects::SetLocked
         mount_mutation ::Mutations::Iterations::Create
         mount_mutation ::Mutations::Iterations::Update
-        mount_mutation ::Mutations::Iterations::Delete
         mount_mutation ::Mutations::Iterations::Cadences::Create
         mount_mutation ::Mutations::Iterations::Cadences::Update
         mount_mutation ::Mutations::Iterations::Cadences::Destroy