Improve handling of deprecated attributes; allow for replacement via WP-CLI

[jeremyfelt]

A few things here:

• On sites where CAB has been active for a long time, mainly one, it's possible that customTaxonomy is a string or an array.
• Also on those sites, it's possible that a block has both the customTaxonomy and taxonomies attributes added.
• The customTaxonomy attribute itself has been deprecated for a while.

So - I think this PR starts setting us up to remove these entirely—primarily because we're really the only ones using this plugin. I've added temporary code to upgrade these attributes in place both in the editor and on the front-end. I've also added a WP-CLI command that we can use to fix the content all at once.

Ideally, after we run wp cab fix-block-attributes on sites that need it (after checking wp cab audit-blocks), then we can remove all of the fallbacks and rely only on the taxonomies attribute.

At the same time I noticed that customPostType is/was being stored like document,document, etc... and causing the selection box to default to "Posts" even when document was selected. I don't think we actually rely on that structure, so I've updated that in the WP-CLI command as well.

[github-actions]

Changed files were detected after build. Please run project build workflows and check git status for any missed changes.

 M build/content-aggregator.asset.php
 M build/content-aggregator.js

You may need to update your local NPM configuration if different than below:

• Node v18.19.0
• NPM 10.2.3

[philcable]

Looks good!