Issue Summary

This is reproducible on Data Center: yes

When implementing app custom field migrations the mappings are retrieved under `jira/classic:appCustomField` in the latest migration platform this is no longer the case and is published under `jira/classic:customField` which causes a breaking change.

Steps to Reproduce

1. Implement app custom field migration https://developer.atlassian.com/platform/app-migration/tutorials/migrating-app-custom-fields/#step-1:-server-app-provides-the-app-custom-field-mappings-to-be-migrated 
2. Migrate a project with issues that have the app custom field to be migrated with JCMA and latest migration platform
3. Attempt retrieve the mappings under `jira/classic:appCustomField` but no mappings are returned

Expected Results

Mappings are returned under `jira/classic:appCustomField` for migrated app custom fields

Actual Results

No mappings returned

Workaround

Retrieve mappings under  `jira/classic:customField` until this is fixed.