Remove native notification services

[cafferata]

I wanted to propose removing the native notification services (Slack/Mattermost, Microsoft Teams, Telegram) in favor of fully transitioning to Shoutrrr. Shoutrrr provides broader support and flexibility, making it a more streamlined and unified solution for notifications. Let me know what you think!

[tlimoncelli]

YES!

Take it a bit further:

1. creds.js shouldn't need notifications -> shouterrr_url. There should be a "shouterrr" option that just takes a list of shouterrr URLs.
2. Native dnscontrol notifications should rewrite the URL into a shouterrr URL so we can delete the old code.

[cafferata]

Originally posted by @tlimoncelli in #3242 (comment)

If there are any methods that are supported by both the native dnscontrol and Shoutrrr, let's mark the native one as "legacy" and recommend the Shoutrrr version in the future. (In fact, maybe in the future the native methods should just call Shoutrrr?)

[hmoffatt]

Sounds good to me.