Install guide RHEL instructions are invalid on RHEL 7

The instructions in the install guide for RHEL systems are specific to RHEL 8, but make no mention of that fact. We should either write up a separate path for RHEL 7 or, as a bare minimum, add a summary warning at the top of the RHEL section.

The relevant differences between RHEL 7 and RHEL 8 include:

• Need to use yum on RHEL 7 (not dnf) to install packages (EDIT: as Dino points out, we could just say yum for both, since it's aliased to dnf on RHEL 8)
• The URL for installing the repository RPM needs to be changed
• Additional work is required to install PostgreSQL 10 or newer on RHEL 7 (whereas RHEL 8 ships with PostgreSQL 10 by default)

I encountered this issue in the course of helping a support customer get a new lab system set up.