Use GitHub script to handle JSON escaping in Slack message

[dhruvkb]

Fixes

Fixes #3781 by @obulat

Description

This PR updates the Slack notification workflow to use a JSON file written by a actions/github-script step. This should handle all issues related to JSON escaping that previous attempts could not fix. It also combines the new and undraft steps into a single step that can handle both scenarios.

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[obulat]

LGTM, hope this finally fixes the problem

[sarayourfriend]

LGTM!