Add rate limit for DELETE deployments endpoint

What does this MR do and why?

Add rate limit for DELETE deployments endpoint

Introduce a dedicated 500 req/min per-user rate limit for DELETE /api/v4/projects/:id/deployments/:deployment_id using Gitlab::ApplicationRateLimiter.

The endpoint previously had no specific limit, only inheriting the global Rack::Attack project ceiling of 2,000 req/min.

Register the key in ApplicationRateLimiter and the Labkit adapter, check it before processing the delete, and add a rate-limited endpoint spec covering per-user scope isolation.

References

https://gitlab.com/gitlab-com/gl-infra/production-engineering/-/work_items/28426+

How to set up and validate locally

1. Create a project with a couple of deployments (via the API or
   in the Rails console).
2. (Optional, to avoid sending 500 requests) temporarily lower the
   threshold in the Rails console:

   allow(Gitlab::ApplicationRateLimiter)                                                                                                            
     .to receive(:threshold).with(:deployment_delete).and_return(2)                                                                                 
   or edit deployment_delete in                                                                                                                     
   lib/gitlab/application_rate_limiter.rb to a small value.                                                                                         

3. Delete deployments past the threshold as the same user:
   curl --request DELETE \
   --header "PRIVATE-TOKEN: <your_access_token>" \
   --url "http://127.0.0.1:3000/api/v4/projects//deployments/<deployment_id>"
4. The first requests return 204 No Content; once the per-user
   limit is exceeded the endpoint returns 429 Too Many Requests
   with a Retry-After header.
5. Confirm a different user is not throttled (per-user scope).
6. Run the spec:
   bin/rspec spec/requests/api/deployments_spec.rb \
   -e "rate limited endpoint"

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