Update schema and syntax

[lmolkova]

We're moving schema file to the weaver project (as a part of #322 and open-telemetry/semantic-conventions#916).

This PR updates and cleans up schema definition and syntax docs:

• migrate to the latest json-schema version
• require stability on everything
• remove prefix
• remove allow_custom_values on enum attributes
• allow deprecation on groups

Some of these properties were removed from build-tools code, others (like allow_custom_values) are still supported by the tooling.

Schema is not enforced: removing properties from the schema does not break anything, it will just improve VSCode autocomplete and highlight unknown properties in red.

TODO:

• changelog