Run tests with YAML metaschemas

[ralfhandl]

The schema-test workflow was using the JSON metaschemas, whereas the README.md files claim that the YAML files are the source of truth.

• Use YAML metaschemas in schema tests
• Use YAML metaschemas in validate.mjs
• Remove out-of-sync JSON files
• Update README.md files

[mikekistler]

Looks good. 👍

One minor comment.

[handrews]

I'd still prefer to not have the JSON on main at all. I think the problem of "these schemas look published but they are not" is the bigger problem here, and I don't think we should invest in the current arrangement.

Per #3677, I think we should focus on setting up schema development correctly on the new dev branch (and release-line-specific branches off of it), which is where all work-in-progress will live.

If there's an argument that this is a necessary step, I'm open to it, but this feels like going backwards to me by trying to bless main as an alternate publication location.

[ralfhandl]

I'd still prefer to not have the JSON on main at all

The JSON is used by the schema tests on main. We can

1. Generate the JSON on the fly the YAML before running the tests, or
2. Change the tests to read the YAML instead

In addition we can remove the JSON from main.

I think we should focus on setting up schema development correctly on the new dev branch

Fine with me, but the schema tests being based on the JSON contradicts the README's claim that the YAML are is leading, and fixing that now makes it easier to move the sources & scripts somewhere else.