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.

This changes the guides for upgrading from CE source to EE source so we
no longer need separate documents for every version. This removes the
need for release managers to create and update these documents, and
allows developers to add these instructions as part of their code
changes.

Instead of requiring one separate document for every version (which is
created by release managers), we now use a single document for both
Community Edition and Enterprise Edition. This allows developers to add
guidelines right away, instead of release managers having to determine
what to add.

Version specific upgrading instructions are placed in a separate
section. This prevents instructions from lingering around for many
versions should a developer forget to remove them. This also ensures
that all instructions are kept in a single place, instead of being
spread across different documents.

A minor downside is that CE and EE guidelines now live in the same
document, which could cause merge conflicts. Since we are working
towards a single codebase, this should only be an issue until we merge
the codebases together; something we expect to do in the coming months.

This commit also removes all old upgrading instructions, but these can
still be accessed using Git branch specific URLs (included in the
updating README).

Avoid installing Bundler 2.0 or higher on source-based install

Closes #55979

See merge request gitlab-org/gitlab-ce!24502