Fix RuboCop violations for task helpers

What does this MR do and why?

Fix RuboCop violations for task helpers so we can eventually turn on routine RuboCop testing.

Fixes the following violations:

• Style/RedundantBegin: https://docs.rubocop.org/rubocop/cops_style.html#styleredundantbegin
• Lint/NoReturnInBeginEndBlocks: https://docs.rubocop.org/rubocop/cops_lint.html#lintnoreturninbeginendblocks

The updated:

• products function is used in clone_repositories Rake task, which is called as part of bundle exec rake default in the compile_dev job.
• stable_branch_name function is used in the retrieve_branch function, which is also used in the clone_repositories Rake task, which is called as part of bundle exec rake default in the compile_dev job.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.