Release Management: create post-release version bump workflow

When releasing Logstash, one of the post-release tasks is to bump the Logstash core version so that subsequent snapshot builds and DRA artifacts include the _next_ version for the branch (e.g., when `8.17.2` was released today, we updated the version on the `8.17` branch to `8.17.3`, causing the next snapshot build to be `8.17.3-SNAPSHOT`).

We should have a workflow to handle this bit _for_ us.

The task, as paraphrased for the `8.17.2` release example above, is:

> ~~~ sh
> # check out a new branch based on our release branch
> git checkout origin/8.17
> cp Gemfile.template Gemfile
> rake version:set[8.17.3]
> ~~~

Notably:
- the person doing the work of releasing `X.Y.Z` needs to check out branch `X.Y` and set the version to `X.Y.(Z+1)`.
- We want this job to be idempotent (that is: if run multiple times we don't want it to bump the version multiple times).

Therefore, the job should have the version _being released_ as its input. From the version being released, we can derive both the branch name and the version we are bumping _to_.

Since this is a pre-requisite for kicking off fresh staging and DRA-artifact builds so that they can get the new version, it would be nice to also trigger those.