automate monthly issue

Production checks rely on the current milestone to identify the monthly issue in order to log the check results.

%13.1 will expire June the 17th, starting from the 18th auto-deploy branches will start with 13-2-auto-deploy

Monthly release preparation is a manual chatops action at the moment, but we need to have it ready when we deploy on production June the 18th.

Because of unclear milestone change dates gitlab-com/gl-infra/delivery#810 (closed) will be better to tie monthly issue preparation to auto-deploy branch preparation, in this way we can later tweak milestone expirations and this will still work.

What's included in this MR

Our rake files are a bit too complex, here I started a refactoring that moves rake tasks to a command pattern into lib/release_tools/tasks/**/*.rb.

1. Refactor rake auto_deploy:prepare into lib/release_tools/tasks/auto_deploy/prepare.rb
2. Move all global task_helper into a module in ``lib/release_tools/tasks/helper.rb`
3. Refactor rake release:prepare into lib/release_tools/tasks/auto_deploy/prepare.rb
4. Refactor rake release:issue into lib/release_tools/tasks/auto_deploy/issue.rb
5. Invoke ReleaseTools::Tasks::Release::Prepare from ReleaseTools::Tasks::AutoDeploy::Prepare