improve release notes flow

[jamieklassen]

• store release notes in concourse/concourse
• write release notes in markdown instead of booklit
• remove any CI pipeline references to concourse/release-notes
• destroy the concourse/release-notes repo

[jomsie]

Thinking about the new flow re: release notes.

Since the release notes md files are in concourse/release_notes, we were thinking of having a latest.md file in there, and whenever anyone makes a PR to concourse/concourse, they'd also add their release note in there.

Then once it's time to release a new version of concourse, before the shipit job, the releng pair will have to copy latest.md to v{version}.md, clear out latest.md, and then make a PR with those changes.