Migration Guide: mention that OSGi can't be migrated to EAP 7

EAP 6 had OSGi support, initially as Tech Preview, but was demoted to Unsupported in 6.1.0 (based on looking through the EAP 6.x release notes history). The EAP 6 standalone configs only had the configadmin and osgi subsystems/extensions in a separate file standalone-osgi.xml, but the default EAP 6 domain config domain.xml has the configadmin extension/subsystem always defined. And since we ship standalone-osgi.xml even with EAP 6.4, I think a short mention in the Migration Guide is appropriate.

This can't be migrated to EAP 7, because EAP 7 simply no longer ships the code (actually it does have the code for the configadmin extension/subsystem, because of managed domain, but it doesn't have the code for the osgi extension/subsystem). Given that it's been unsupported in EAP 6 for so long, I think a very short notice would be perfectly fine. The biggest reason for adding a short chapter about this is that it's a realistic migration concern for migrating a managed domain.