add deprecation warning for CentOS 7 / RHEL 7

[thaJeztah]

• relates to https://github.com/docker/runtime-team/issues/119
• relates to https://github.com/docker/docker-install/pull/427

With this patch:

sh -c install.sh
# Executing docker install script, commit:

DEPRECATION WARNING
    This Linux distribution (centos 7) reached end-of-life and is no longer supported by this script.
    No updates or security fixes will be released for this distribution, and users are recommended
    to upgrade to a currently maintained version of centos.

Press Ctrl+C now to abort this script, or wait for the installation to continue.