Clarify "Skip Users" option in CCMA

Issue Summary

When using the Confluence Cloud Migration Assistant, it is not obvious the "Skip Users" option is only available for the "Migrate Attachments" option.

Steps to Reproduce

1. Create a new migration plan in CCMA
2. Select "Migrate spaces" option
   Observe "Skip users" option is disabled.

• After selecting "Migrate attachments", observe that "Skip users" is enabled.

Suggestion

Add ancillary text to clarify that the "Skip users" option is only available when selecting the "Migrate attachments" option, e.g. 'Skipping user migration is only possible when selecting the "Migrate attachments only" option.'