Use canonical Slack Manifest JSON schema
What does this MR do and why?
This MR updates a hand-made JSON Schema file for a Slack manifest with a canonical one.
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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I have evaluated the MR acceptance checklist for this MR.
Related to #358872 (closed)