It looks like I've got to implement a number of changes for Gutenberg 10.6

1. Switch to theme.json
2. Implement post-terms blocks for categories and tags
3. Fix the horizontal menu styling

Switch to theme.json

In Gutenberg 10.6, the theme JSON file no longer be experimental.
It's been restructured.

We'll need a theme.json file for Gutenberg 10.6 and above with "version": 1.

And experimental-theme.json for earlier versions of Gutenberg, though this won't be maintained.

See https://github.com/WordPress/gutenberg/blob/trunk/docs/how-to-guides/themes/theme-json.md

Implement post-terms blocks for post categories and tags

There have been other changes.
post-tags and post-hierarchical-terms have become two variations of post-terms.

Fix horizontal menu styling

The menu when looking at a single-post was:

It's now.:

• Lost a lot of padding
• Everything's blue.
• With unwanted underscoring

or is it? The fonts have gone tits up again. Must've been my attempt to put templateParts into theme.json without anything else!