Issue Summary

In Jira server to cloud migration, we don't support the Text Field(read only) custom fields migration.

The field is not migrated and we see the below message in post-migration report

The "Text Field (read only)" custom field type is not supported via migration. The configuration for this field will not be migrated and any items that reference it, such as workflow schemes and filters may no longer work. 

Workaround

Use the CSV import method as described in below article:
How to bulk import custom field values for certain types of Custom Field