Updated content around message status callbacks

[jamestynan-content]

Description

Removed the following statuses and corresponding descriptions in 'Message status' section on Message channel supplier status page:

• pending_enrichment
• enriched
• sending

Removed reference to choosing message status on the Onboard with Notify page.

Context

Previously, a new integrator could choose which message status they want to receive as part of their onboarding setup.

This process has now changed - we only offer two message statuses ('delivered' and 'failed') as default. A user can no longer choose pending_enrichment, enriched or sending statuses.

These changes reflect the new process.

Type of changes

• [x ] Refactoring (non-breaking change)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would change existing functionality)
• Bug fix (non-breaking change which fixes an issue)

Checklist

• [x ] I am familiar with the contributing guidelines
• [x ] I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

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• I confirm that neither PII/PID nor sensitive data are included in this PR and the codebase changes.

[emmagifford]

Changes all look good to me!