Update stop-staging-slots.yml secret name

[mimartin12]

Type of change

- [X] Bug fix
- [ ] New feature development
- [ ] Tech debt (refactoring, code cleanup, dependency upgrades, etc)
- [X] Build/deploy pipeline (DevOps)
- [ ] Other

Objective

Updates the secret name in the staging slot workflow.

Code changes

• .github/workflows/stop-staging-slots.yml: Update secret name.

Before you submit

• Please check for formatting errors (dotnet format --verify-no-changes) (required)
• If making database changes - make sure you also update Entity Framework queries and/or migrations
• Please add unit tests where it makes sense to do so (encouraged but not required)
• If this change requires a documentation update - notify the documentation team
• If this change has particular deployment requirements - notify the DevOps team