Remove create commands

[isaacroldan]

WHY are these changes introduced?

As part of the CLI 2.x sunset, we are removing the commands to create an app or extension.
Contributes to https://github.com/Shopify/internal-cli-foundations/issues/610

WHAT is this pull request doing?

Remove app create command
Remove extension create command

How to test your changes?

Post-release steps

Update checklist

• I've added a CHANGELOG entry for this PR (if the change is public-facing)
• I've considered possible cross-platform impacts (Mac, Linux, Windows).
• I've left the version number as is (we'll handle incrementing this when releasing).
• I've included any post-release steps in the section above (if needed).

[gonzaloriestra]

LGTM, but there are some tests to fix.