Remove ign CLI from version 2

[chapulina]

Summary

Initially we intended to provide users with a tick-tock of the CLI, so they could use the ign CLI with Garden+ and see warnings. But this means Garden+ wouldn't be co-installable with Citadel and Fortress. Given the option to either:

1. Support side-by-side installation of Citadel/Fortress and Garden
2. Tick-tock the CLI and make a hard change from ign to gz

We've decided to go with 1. The reasoning is that users should quickly realize that they need to use the new tool. Unlike namespaces and headers, the CLI isn't hardcoded in many places and is mostly invoked by hand, so tick-tock is less important there. On the flip side, side-by-side installation is helpful for migration and mixed systems.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

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