Updated documentation as per latest missing features on about.gitlab.com

See merge request gitlab-org/gitlab-ee!5794

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This updates the requirements guide to include a link to an issue where
we discuss dropping MySQL support for nested groups. This should give
users a better understanding of why we highly recommend them to use
PostgreSQL instead.

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This makes it more obvious that we greatly prefer PostgreSQL over
MySQL/MariaDB and that all users are recommended to use PostgreSQL
instead. This also removes the paragraph mentioning the size per user
row, as this is completely inaccurate and doesn't really help users with
anticipating the database size. Instead we now simply recommend users to
have at least 5-10 GB of storage at minimum.

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