Job status of jobs executed before the upgrade to 16.10.x cannot be loaded anymore

Steps to reproduce:

1. Install an XWiki version < 16.10.0RC1, like 16.9.0
2. Perform a job like installing an extension.
3. Upgrade to XWiki 16.10.0 (or 16.10.4)
4. Try accessing the job status, either directly through the REST API or by checking in the extension manager if there is a "Progress" tab when clicking on the extension in the "History".

Expected result:

The job status can be retrieved.

Actual result:

The job status cannot be retrieved. This is because XCOMMONS-2387 changed the directory names of the job status to be base64-encoded without migrating any existing job status.