CONTRIBUTING.md: Drop CentOS 6 note
CONTRIBUTING.md: Drop CentOS 6 note

CentOS 6 is EOL by the end of November 2020. We can already remove it
from our example documentation.

dhoppe/modulesync_config,dhoppe/modulesync_config,puppet-community/modulesync_config,voxpupuli/modulesync_config,dhoppe/modulesync_config,voxpupuli/modulesync_config,dhoppe/modulesync_config,voxpupuli/modulesync_config,voxpupuli/modulesync_config