Expand `rake version:sync` to also update asciidoc variables and the README.md

Update the documentation and the readme file is done manually by a human and this is error prone.
We already have a rake task that take care of updating the version of the gems and also the dependencies requirements called `rake version:sync`.

We can expand this task to also take care of updating the following:

- [ ] The links in the [README.md](https://github.com/elastic/logstash/blob/master/README.md)
- [ ] The shared variables in the (asciidoc)[https://raw.githubusercontent.com/elastic/logstash/master/docs/index-shared1.asciidoc

Start with the existing `rake version:sync` task and inspect how it updates gem versions and dependency requirements. Then review `README.md` and `docs/index-shared1.asciidoc` to identify the links and shared variables that should be synchronized. Done means the task updates both documentation locations consistently when the version changes.

Written by the indexing model from the issue text.