docs: update GitLab security docs + announce Mend-hosted GitLab.com

[jamietanna]

Changes

Mend's GitLab.com support has been around for a little bit, but we've
not explicitly documented it.

While we're doing this, we should make sure the outdated references to
GitLab security is updated in line with the current state of affairs.

Context

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• This closes an existing Issue, Closes: #
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