Adds 16.11 to archived versions

We'll keep this version (16.11) long-term because it's the last 16.x version, but first we have to add it here.

Related items

• Related to docs.gitlab.com release 17.2 (July, 2024) (gitlab-docs#1796 - closed) where I'm doing the docs release this month.
• Related to Revise the previous-version step for clarity (gitlab-docs!4903 - merged) where I'm improving the instructions for this step because I was glassy-eyed.