Mention limitations of MySQL in docs
What does this MR do?
Document the limitations of MySQL and why users should switch using PostgreSQL instead.
Mostly ticks the checkboxes in https://gitlab.com/gitlab-org/gitlab-ce/issues/30472#note_29072566
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Document the limitations of MySQL and why users should switch using PostgreSQL instead.
Mostly ticks the checkboxes in https://gitlab.com/gitlab-org/gitlab-ce/issues/30472#note_29072566