Use a dist README fixture kept in the jQuery repository instead of modifying
an existing one. This makes the jQuery repository the single source of truth
when it comes to jQuery releases and it makes it easier to make changes to
README without worrying how it will affect older jQuery lines.
The commit also ES6ifies build/release.js & build/release/dist.js
Closes gh-4614
(cherry picked from commit 358b769a00)
