Enable frozen string in lib/api and lib/backup

Partially addresses #47424.

Had to make changes to spec files because
stubbing methods on frozen objects is a mess
in RSpec and leads to failures:

https://github.com/rspec/rspec-mocks/issues/1190

changelogs/unreleased/lib-api-frozen-string-enable.yml

+---
+title: Enable frozen string in lib/api and lib/backup
+merge_request:
+author: gfyoung
+type: performance

lib/api/access_requests.rb

+# frozen_string_literal: true
+
 module API
   class AccessRequests < Grape::API
     include PaginationParams

lib/api/api.rb

+# frozen_string_literal: true
+
 module API
   class API < Grape::API
     include APIGuard

lib/api/api_guard.rb

+# frozen_string_literal: true
+
 # Guard API with OAuth 2.0 Access Token
 require 'rack/oauth2'

lib/api/applications.rb

+# frozen_string_literal: true
+
 module API
   # External applications API
   class Applications < Grape::API

lib/api/avatar.rb

+# frozen_string_literal: true
+
 module API
   class Avatar < Grape::API
     resource :avatar do

lib/api/award_emoji.rb

+# frozen_string_literal: true
+
 module API
   class AwardEmoji < Grape::API
     include PaginationParams

lib/api/badges.rb

+# frozen_string_literal: true
+
 module API
   class Badges < Grape::API
     include PaginationParams

lib/api/boards.rb

+# frozen_string_literal: true
+
 module API
   class Boards < Grape::API
     include BoardsResponses

lib/api/boards_responses.rb

+# frozen_string_literal: true
+
 module API
   module BoardsResponses
     extend ActiveSupport::Concern

lib/api/branches.rb

+# frozen_string_literal: true
+
 require 'mime/types'
 module API

lib/api/broadcast_messages.rb

+# frozen_string_literal: true
+
 module API
   class BroadcastMessages < Grape::API
     include PaginationParams

lib/api/circuit_breakers.rb

+# frozen_string_literal: true
+
 module API
   class CircuitBreakers < Grape::API
     before { authenticated_as_admin! }

lib/api/commit_statuses.rb

+# frozen_string_literal: true
+
 require 'mime/types'
 module API

lib/api/commits.rb

+# frozen_string_literal: true
+
 require 'mime/types'
 module API

lib/api/custom_attributes_endpoints.rb

+# frozen_string_literal: true
+
 module API
   module CustomAttributesEndpoints
     extend ActiveSupport::Concern

lib/api/deploy_keys.rb

+# frozen_string_literal: true
+
 module API
   class DeployKeys < Grape::API
     include PaginationParams

lib/api/deployments.rb

+# frozen_string_literal: true
+
 module API
   # Deployments RESTful API endpoints
   class Deployments < Grape::API

lib/api/discussions.rb

+# frozen_string_literal: true
+
 module API
   class Discussions < Grape::API
     include PaginationParams

lib/api/entities.rb

+# frozen_string_literal: true
+
 module API
   module Entities
     class WikiPageBasic < Grape::Entity