release-notes: structure: update 6.4.x
release-notes: structure: update 6.4.x

- add attribution for community PRs
- re-word release note for step refactor to include details about
  stricter validation

Signed-off-by: Sameer Vohra <9dd3a30774efbd15e2401a4c8ee8d5219d46192f@gmail.com>

concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse