Now MySQL is no longer supported, we need to change the docs

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).

Originally submitted at
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6004

.

Signed-off-by: Rémy Coutable <remy@rymai.me>

The introduction of background migrations means some of our requirements
for online upgrades have to be adjusted / clarified.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/35939 for more
information.

Starting with GitLab 9.1.0 we will no longer allow downtime migrations
unless absolutely necessary. This commit updates the various developer
guides and adds code that is necessary to make zero downtime migrations
less painful.

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This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

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Rephrased wording in the documentation to say "installation from source" instead of  "manual installation" or similar.

product names, plus a couple of instances of bad Markdown markup.