Remove Flowdock integration

Description

https://github.com/flowdock/flowdock-api is present in our gemfile.
We also have an integration described in docs https://docs.gitlab.com/ee/api/integrations.html#flowdock

At the same time:

• Flowdock has been shut down https://twitter.com/flowdock/status/1554231548873912320?cxt=HHwWgMCtlf3W3pErAAAA
• http://www.flowdock.com/ is also down, refer to their Twitter for other details
• the gem is unmaintained

Tasks

Based on #27954 (closed).

15.7

• Remove Flowdock integration !102394 (merged)
  • Code (model/controller/service/view/api)
  • Remove the gem
  • Remove Docs
    • Reference: https://docs.gitlab.com/ee/api/integrations.html#flowdock
  • Remove usage ping metric https://gitlab.com/gitlab-data/analytics/-/issues/15023
    • Reference: https://docs.gitlab.com/ee/development/service_ping/metrics_lifecycle.html#remove-a-metric
• Add a removal entry !105345 (merged)
  • Reference: https://docs.gitlab.com/ee/development/deprecation_guidelines/#update-the-deprecations-and-removals-documentation-pages
• Remove Flowdock database records from the integrations table !105363 (merged)

  DELETE FROM integrations WHERE type_new = 'Integrations::Flowdock'

  Integration.each_batch(of: 100_000, column: :id) do |relation|
    relation.delete_by(type_new: 'Integrations::Flowdock')
  end

15.8

• Remove extra code: Flowdock dummy model and/or extra code !108032 (merged)
• Remove ~"Integration::Flowdock" issues, merge requests, epics and label