Deprecate MySQL support for Omnibus EE in docs

• Add a note that MySQL support for Omnibus EE is deprecated
• link to its limitations https://docs.gitlab.com/ce/install/requirements.html#database

https://docs.gitlab.com/omnibus/settings/database.html#using-a-mysql-database-management-server-enterprise-edition-only

marked this issue as related to gitlab-com/www-gitlab-com#1547 (closed)

mentioned in issue gitlab-com/www-gitlab-com#1547 (closed)

mentioned in merge request !1756 (merged)

marked the checklist item link to its limitations https://docs.gitlab.com/ce/install/requirements.html#database as completed

marked the checklist item Add a note that MySQL support for Omnibus EE is deprecated as completed

closed via commit 07c42bae

mentioned in commit 07c42bae