Add draft changelog for v5
Add draft changelog for v5

As part of getting ready to release our current main branch to the world
as a release candidate (and subsequently a v5.0.0) I've collected
together as many changes as I could find, highlighting what's new and
what potentially breaking changes there are.

holmesjr/elastic-ci-stack-for-aws,holmesjr/elastic-ci-stack-for-aws,buildkite/elastic-ci-stack-for-aws