Remove stdout and stderr shorthands in base command

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What does this merge request do and why?

In !2103 (merged), we replaced the stdout and stderr in GDK::Command::BaseCommand with GDK::Output instead of the $stdout/$stderr globals. Therefore, we want to use GDK:Output inside lib/gdk/command/* instead of stdout and stderr.

How to set up and validate locally

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

For the changes to debug_info.rb

1. Run gdk debug-info. The debug information should be printed without errors.

For the changes to diff_config.rb

1. Make changes to your config. For example, run gdk config set postgresql.port 1.
2. Run gdk diff-config. The changes to your configuration should be shown without errors.

For the changes to base_command.rb

1. Run gdk doctor. The command should terminate without errors.

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.

Closes #1224 (closed)