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 https://gitlab.com/gitlab-org/gitlab/-/issues/27954.
### 15.7
- [x] Remove `Flowdock` integration https://gitlab.com/gitlab-org/gitlab/-/merge_requests/102394
- [x] Code (model/controller/service/view/api)
- [x] Remove the gem
- [x] Remove Docs
- Reference: https://docs.gitlab.com/ee/api/integrations.html#flowdock
- [x] 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
- [x] Add a [removal entry](https://docs.gitlab.com/ee/update/removals.html) https://gitlab.com/gitlab-org/gitlab/-/merge_requests/105345
- Reference: https://docs.gitlab.com/ee/development/deprecation_guidelines/#update-the-deprecations-and-removals-documentation-pages
- [x] Remove `Flowdock` database records from the `integrations` table https://gitlab.com/gitlab-org/gitlab/-/merge_requests/105363
```sql
DELETE FROM integrations WHERE type_new = 'Integrations::Flowdock'
```
```ruby
Integration.each_batch(of: 100_000, column: :id) do |relation|
relation.delete_by(type_new: 'Integrations::Flowdock')
end
```
### 15.8
- [x] Remove extra code: Flowdock dummy model and/or extra code https://gitlab.com/gitlab-org/gitlab/-/merge_requests/108032
- [x] Remove ~"Integration::Flowdock" issues, merge requests, epics and label
issue