feat: common prBody across all types (#2542)

lib/config/definitions.js

lib/config/migration.js

lib/config/templates/default/pr-body.hbs

lib/config/templates/digest/pr-body.hbs

lib/config/templates/docker-digest-group/pr-body.hbs

lib/config/templates/docker-digest/pr-body.hbs

lib/config/templates/docker-group/pr-body.hbs

lib/config/templates/docker-pin-group/pr-body.hbs

lib/config/templates/docker-pin/pr-body.hbs

lib/config/templates/docker/pr-body.hbs

lib/config/templates/group/pr-body.hbs

lib/config/templates/lock-file-maintenance/pr-body.hbs

lib/config/templates/node/pr-body.hbs

lib/config/validation.js

lib/workers/pr/changelog/hbs-template.js

lib/workers/pr/index.js

lib/workers/pr/pr-body.js

test/config/__snapshots__/index.spec.js.snap

test/config/__snapshots__/migration.spec.js.snap

test/config/migration.spec.js