bootstrap/cbuild: better detect centos 7 (and older)

There are various well-known build tags for CentOS 7 and older. According to

  $ skopeo inspect docker://quay.io/centos/centos:8

we see for example

• "7"
• "7.9.2009"
• "centos7.9.2009"
• "centos7"

Better detect such CentOS versions.