Ruby 2.6 offers better performance and memory usage:
https://www.rubyguides.com/2018/11/ruby-2-6-new-features/

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/57323

This change refactors the pgloader instructions to have
less headings and be more straightforward.

Update the documented version requirement of Ruby inside of Upgrading
from Source to show GitLab 11.8 needs Ruby 2.5

Markdown renderers find it easier to determine
where lists start and end when lists are surrounded
by new lines.

For consistency, also ensure entries in the list
are aligned when they span multipls lines.

For the sake of consistency, removes any extraneous
consecutive blank lines from the doc suite.