PORT-8073 | Update Self Service Action for Creating Slack Channel

[mk-armah]

Description

Replaced port_payload with similar feature that sends essential port_context via the action input. This to take advantage of the control the payload feature of Port actions.

Updated docs pages

create-self-service-experiences/setup-backend/github-workflow/examples/open-slack-channel

[aws-amplify-eu-west-1[bot]]

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