Add install and update for centos/rhel 8

Why is this change being made?

Packages for centos 8 were introduced in GitLab 12.8, ensuring we have instructions for them here.

Adds install information for centos8/rhel8. This was added as a separate item from the existing centos 7 entry, as the dependency information for centos8 is different.

The new instructions also take advantage of the new dnf package tool, rather than yum. Though this isn't necessary.

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what