[PP-1] Remove schema references from contrib in SDC tests in core

Interesting question! So what "is" a schema and where should it live?
Some thoughts:
- If the validator can eat a URL from the test instnce, then it always gets the correct one.
- Gitlab does have "eternal" URLs like https://git.drupalcode.org/project/sdc/-/raw/HEAD/src/metadata.schema.json
- Can a schema change in a first place? Probably yes. But if so, we should apply sth like Semver to it, increase major version for non BC breaks, and have URLs containing major version, like: HOST/core/sdc/v1/metadata.schema.json

I like the idea of using HEAD and a version identifier in the URL.

I added a note in 📌 Move SDC *.schema.json to Core namespace Active to make that happen. I am postponing this issue until that happens.