[skip ci] Automate discussion template updates with weekly workflow

The discussion template requires manual updates whenever new RabbitMQ or
Erlang versions are released. This maintenance burden increases with the
frequency of releases and can lead to outdated version lists in the
template.

This change introduces a weekly GitHub Actions workflow that
automatically fetches the latest versions and updates the discussion
template. The workflow uses `gh release list` to fetch stable RabbitMQ
4.x releases, filters to the last 2 minor versions, and fetches Erlang
versions from the OTP versions table, filtering to the last 3 major
versions by minor.

The implementation uses YTT templating to generate the discussion form
from `questions.yml.tpl`, with version data stored in `versions.json`.
The workflow creates a pull request only when versions change, assigns
it to maintainers for review, and uses `[skip ci]` to avoid unnecessary
CI runs.

The automation runs weekly on Mondays at 9am UTC and can be manually
triggered via workflow dispatch.

Author: lukebakken