fix: replace non-existent labels with valid repository labels

[lgallard]

Summary

Fixes the automated issue creation failure in the AWS Backup Feature Discovery workflow by replacing invalid labels with valid repository labels.

Problem

The workflow was failing with label validation errors:

• aws-provider-update - label doesn't exist
• auto-discovered - label doesn't exist
• needs-triage - label doesn't exist

This prevented automated GitHub issue creation when new AWS Backup features were discovered.

Solution

Replaced invalid labels with valid repository labels:

• enhancement - already valid
• aws-backup - exists in repository
• features - exists in repository
• terraform - exists in repository

Test plan

• Verify labels exist in repository (gh repo view --json labels)
• Test label combination works with issue creation
• Run workflow end-to-end to confirm automated issue creation succeeds
• Verify all 5 pending discovered features get processed correctly

Impact

Enables automated GitHub issue creation for discovered AWS Backup features, completing the feature discovery workflow functionality.