fix: use INCOMING_WEBHOOK for slack hook

[takirala]

set the type explicitly for richer formatting

This should be the final fix after #2819

What problem does this PR solve?:

Which issue(s) does this PR fix?:

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Checklist

• If the PR adds a version bump, ensure there is no breaking change in Licensing model (or NA).
• If a chart is changed or app configuration is significantly changed, the chart version is correctly incremented (so that apps are not automatically upgraded from a previous version of DKP).

[coveralls]

Pull Request Test Coverage Report for Build 11828166629

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 51.703%

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Totals
Change from base Build 11825055115:	0.0%
Covered Lines:	167
Relevant Lines:	323

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💛 - Coveralls