Add IP fallback errors to Gitlab::HTTP HTTP_ERRORS

What does this MR do and why?

• Adds Errno::EADDRNOTAVAIL, Errno::EAFNOSUPPORT and Errno::ETIMEDOUT to Gitlab::HTTP_V2::HTTP_ERRORS (which Gitlab::HTTP::HTTP_ERRORS points to). These errors trigger the next-IP fallback added in !239733 (merged), but they were not in the list. When the last IP also failed, callers that rescue *Gitlab::HTTP::HTTP_ERRORS did not catch the error.
• Webhooks: WebHookService did not rescue the error, so the failure was not logged in the hook's logs, the hook did not get the normal backoff or auto-disable, and Sidekiq retried the job.
• CI include:remote: pipelines failed with a misleading "Unable to run pipeline - Undefined error".
• Adds a spec for each fallback error, checking that Gitlab::HTTP_V2.try_get returns nil when every resolved IP fails with that error.

References

• #344788 (closed)
• #630166
• !239733 (merged)
• https://new-sentry.gitlab.net/organizations/gitlab/issues/GITLABCOM-ERSC

Expected side effects

• Webhooks that fail on every attempt with these errors now count as failures (rescue), so they get backed off and eventually auto-disabled. Previously they crashed before the failure was recorded.
• Audit event streaming treats these errors as destination failures: they are logged instead of sent to Sentry, and they count toward the destination's circuit breaker.

How to set up and validate locally

1. From gems/gitlab-http, run bundle exec rspec spec/gitlab/http_v2/ip_fallback_spec.rb.
2. Optionally, revert the change in gems/gitlab-http/lib/gitlab/http_v2/exceptions.rb and rerun. The new examples for ETIMEDOUT, EADDRNOTAVAIL and EAFNOSUPPORT fail.

MR acceptance checklist

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