refactor: Add warning to Pin PRs that upgrade PRs will be held up

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

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

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

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

test/manager/__snapshots__/resolve.spec.js.snap

test/workers/pr/__snapshots__/index.spec.js.snap