Update the changelog for v1.18

[willhickey]

Summary of Changes

• Advanced the channels (should have been done when I branched v1.17 🤦)
• Update the instructions
  • Added missing instruction
  • Re-ordered instructions so they match the order of the parts of the log they relate to

[t-nelson]

lgtm

will this be well-formed enough that we could enforce that the changelog conforms with the current channel configuration?

[willhickey]

will this be well-formed enough that we could enforce that the changelog conforms with the current channel configuration?

Yes. I've been pondering exactly what that should look like, and the best way to automate the maintenance here. We obviously wouldn't update the changelog in obsolete branches. There's a judgement call whether to leave out of date info or remove it somewhere during the lifecycle. On #33638 I removed the channel links from the v1.17 version (happy to debate that decision on that PR).

I'll do the maintenance manually for a little longer to dial in the correct behavior before I automate it.