Update security-updates.html.md
Update security-updates.html.md

Rename title to match the Icinga alert.

Add a debug step to find out which upgrades are required.

Link to MySQL upgrade documentation - it may be useful for a dev coming to this page.
alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs