tracking-issue: update roadmap docs, remove availability

Created by: bobheadxi

I noticed that the tracking issue treats roadmap items with an emoji:

This updates the link in the legend to point to what seems to be the latest and greatest docs (the existing link links to a deprecated document), and also removes Availability because anecdotally iteration tracking isn't done with the tracking issue nowadays. It can still be added manually by teams that want it

Test plan

n/a just docs