Fixes attribute on security service

What does this MR do and why?

Issues objects have the web_url attribute to return the belonging URL. The PendingIssuesNotificationService was wrongly using url. This commit updates the serviec to use web_url.

[3] pry(main)> ReleaseTools::GitlabClient.next_security_tracking_issue
=> #<Gitlab::ObjectifiedHash:238480 {hash: {"id"=>123, "iid"=>456, ...}
2025-05-05 14:52:31.009313 D [dry-run] ReleaseTools::GitlabClient -- [HTTParty] [2025-05-05 14:52:31 -0600] 200 "GET https://gitlab.com/api/v4/projects/gitlab-org%2Fgitlab/issues" -
[4] pry(main)> ReleaseTools::GitlabClient.next_security_tracking_issue.url
2025-05-05 14:52:34.954573 D [dry-run] ReleaseTools::GitlabClient -- [HTTParty] [2025-05-05 14:52:34 -0600] 200 "GET https://gitlab.com/api/v4/projects/gitlab-org%2Fgitlab/issues" -
NoMethodError: undefined method `url' for an instance of Gitlab::ObjectifiedHash (NoMethodError)
from /Users/mayra-cabrera/.rvm/gems/ruby-3.3.3/gems/gitlab-5.1.0/lib/gitlab/objectified_hash.rb:43:in `method_missing'
[5] pry(main)> ReleaseTools::GitlabClient.next_security_tracking_issue.web_url
=> "https://gitlab.com/gitlab-org/gitlab/-/issues/537236"

Author Check-list

• Has documentation been updated?