[CentOS] Is every CentOS release supported for 7 years?

> ... I understand now ...

No, you don't.

> is it required to upgrade to each point release in order to continue
> receiving security updates?

It's strictly linear and one-dimensional.

Point releases only mark a specific point in time, where you get a 
little bit more, e.g. additional drivers, an optional new version of 
samba, etc.

You always type 'yum update' and go ahead. Nothing else.