Remove `Spec::Support::Helpers::KeyGeneratorHelper` class

`Spec::Support::Helpers::KeyGeneratorHelper` was added in
857d0391 to generate RSA public key
in openssh format.
At that time we didn't have a gem in Gemfile to do so.

Later, we added `sshkey` gem that allows generating public keys.

Also, we used to have `SSHKeygen` class to generate RSA public keys
in openssh format for specs:

In https://gitlab.com/gitlab-org/gitlab/-/merge_requests/77424,
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/78532
we started using `ssh_data` gem instead of `net-ssh`, `sshkey`,
`net-ssh`, `ed25519`, `bcrypt_pbkdf`, and `SSHKeygen`-class.

We can remove `Spec::Support::Helpers::KeyGeneratorHelper`
in favor of `ssh_data` gem too.

.rubocop_todo.yml

@@ -231,7 +231,6 @@ Naming/HeredocDelimiterNaming:
 Naming/MethodParameterName:
   Exclude:
     - 'lib/gitlab/diff/inline_diff.rb'
-    - 'spec/support/helpers/key_generator_helper.rb'
 
 # Offense count: 218
 # Cop supports --auto-correct.

spec/factories/keys.rb

 # frozen_string_literal: true
 
-require_relative '../support/helpers/key_generator_helper'
-
 FactoryBot.define do
   factory :key do
     title
-    key { Spec::Support::Helpers::KeyGeneratorHelper.new(1024).generate + ' dummy@gitlab.com' }
+    key { SSHData::PrivateKey::RSA.generate(1024, unsafe_allow_small_key: true).public_key.openssh(comment: 'dummy@gitlab.com') }
 
     factory :key_without_comment do
-      key { Spec::Support::Helpers::KeyGeneratorHelper.new(1024).generate }
+      key { SSHData::PrivateKey::RSA.generate(1024, unsafe_allow_small_key: true).public_key.openssh }
     end
 
     factory :deploy_key, class: 'DeployKey'

spec/support/helpers/key_generator_helper.rb

-# frozen_string_literal: true
-
-module Spec
-  module Support
-    module Helpers
-      class KeyGeneratorHelper
-        # The components in a openssh .pub / known_host RSA public key.
-        RSA_COMPONENTS = ['ssh-rsa', :e, :n].freeze
-
-        attr_reader :size
-
-        def initialize(size = 2048)
-          @size = size
-        end
-
-        def generate
-          key = OpenSSL::PKey::RSA.generate(size)
-          components = RSA_COMPONENTS.map do |component|
-            key.respond_to?(component) ? encode_mpi(key.public_send(component)) : component
-          end
-
-          # Ruby tries to be helpful and adds new lines every 60 bytes :(
-          'ssh-rsa ' + [pack_pubkey_components(components)].pack('m').delete("\n")
-        end
-
-        private
-
-        # Encodes an openssh-mpi-encoded integer.
-        def encode_mpi(n) # rubocop:disable Naming/UncommunicativeMethodParamName
-          chars = []
-          n = n.to_i
-          chars << (n & 0xff) && n >>= 8 while n != 0
-          chars << 0 if chars.empty? || chars.last >= 0x80
-          chars.reverse.pack('C*')
-        end
-
-        # Packs string components into an openssh-encoded pubkey.
-        def pack_pubkey_components(strings)
-          (strings.flat_map { |s| [s.length].pack('N') }).zip(strings).join
-        end
-      end
-    end
-  end
-end