Fix: delegate non-Slack URLs to base behavior for SSRF protection
Instead of raising a ValueError for non-matching URLs, delegate to the
base Apprise behavior. This ensures restricted URL checks run properly
for all URLs, maintaining SSRF protections.
Only govcloud URLs (hooks.slack-gov.com) get special handling with the
manual NotifySlack construction and webhook_url override.
Co-Authored-By: Nate Nowack <nate@prefect.io>