Unify formatting of CHANGELOG
Unify formatting of CHANGELOG

- No line breaks within a paragraph (because too often markdown
  renderers are configured to render those as actual line breaks).
- More importantly, use a list everywhere.

Signed-off-by: beorn7 <91e32ae1d26bd312b370cf8378a5a83accca90a0@grafana.com>

prometheus/pushgateway,prometheus/pushgateway,prometheus/pushgateway