[BUG] slacks legacy bot api has now ended. existing code needs to be refactored

[ITJamie]

Description
slack_notify module + state no longer works

Setup
(Please provide relevant configs and/or SLS files (be sure to remove sensitive info. There is no general set-up of Salt.)

Please be as specific as possible and give set-up details.

• on-prem machine
• VM (Virtualbox, KVM, etc. please specify)
• VM running on a cloud service, please be explicit and add details
• container (Kubernetes, Docker, containerd, etc. please specify)
• or a combination, please be explicit
• jails if it is FreeBSD
• classic packaging
• onedir packaging
• used bootstrap to install

Steps to Reproduce the behavior
attempt to use slack_notify module (or slack.post_message state) to send a slack message
this appears in the error log

Failed to send message (legacy_custom_bots_deprecated): notify_ops_slack

this will be the api response when debugged:

{'ok': False, 'error': 'legacy_custom_bots_deprecated', 'context': 'Legacy custom bots has been deprecated on March 31st, 2025. Please create a new app using our updated methods. More info can be found in our changelog: https://api.slack.com/changelog/2024-09-legacy-custom-bots-classic-apps-deprecation'}

Expected behavior
slack post should happen

code affected:
https://github.com/saltstack/salt/blob/v3007.1/salt/utils/slack.py
https://github.com/saltstack/salt/blob/v3007.1/salt/states/slack.py
https://github.com/saltstack/salt/blob/v3007.1/salt/modules/slack_notify.py

Additional context
Add any other context about the problem here.