Handle `Errno::ENETUNREACH` in `Gitlab::HTTP`

What does this MR do and why?

This MR adds Errno::ENETUNREACH as a rescuable error to Gitlab::HTTP::HTTP_ERRORS.

HTTP_ERRORS already handles a related error, Errno::EHOSTUNREACH.

The specific motivation for this change is because we see some Errno::ENETUNREACH exceptions raised within WebHookService (See Sentry issue, internal). Adding this error to Gitlab::HTTP::HTTP_ERRORS will mean the service will successfully disable failing webhooks (from here to Webhooks::LogExecutionService). Other parts of the application that refer to HTTP_ERRORS will also benefit.

MR acceptance checklist

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