refactor: rename changelog fields

lib/manager/_helpers/node/package.js

lib/manager/buildkite/package.js

lib/manager/circleci/extract.js

lib/manager/npm/versions.js

lib/manager/pip_requirements/package.js

lib/workers/pr/index.js

test/manager/buildkite/__snapshots__/package.spec.js.snap

test/manager/circleci/__snapshots__/extract.spec.js.snap

test/manager/npm/__snapshots__/versions.spec.js.snap

...nager/pip_requirements/__snapshots__/package.spec.js.snap