Document or declare the gem module requirement

I hit a module resolution failure when running this role in a clean ansible-core environment.

Observed failure:

- With ansible-core 2.15, the role fails while resolving the bare `gem` module in `tasks/main.yml`.

Error excerpt:

```text
ERROR! couldn't resolve module/action 'gem'. This often indicates a misspelling, missing collection, or incorrect module path.

The error appears to be in 'tasks/main.yml' at the task named:

- name: Install Bundler.
```

This can be hidden in environments that install the full Ansible distribution or already include community collections. For ansible-core-only installs, the role likely needs an explicit documented collection requirement or fully qualified `community.general.gem` module references for the Bundler and configured-gem tasks.

Start with tasks/main.yml, especially the tasks named “Install Bundler” and the configured-gem task, and reproduce the failure with ansible-core 2.15 in a clean environment. Check whether the role documents a collection requirement or uses fully qualified module references; done means the role resolves its gem tasks under ansible-core-only installations.

Written by the indexing model from the issue text.