Change name of security repo

Currently the name of the security repo is .eclipsefdn-private. We are in the process of making this repo public and thus also adjust its name to .eclipsefdn.

The sync script should use the new name as well.

Additionally, the eclipsefdn-security team should be created with public visiblity. Otherwise the team can not be used in CODEOWNERS as only teams that are publically visible and have write access can be used.