[mdatagen] Consider updating metrics `stability` field to match Weaver

[dmitryax]

Problem

Currently, we define stability as a group that typically includes only the level field. The only additional field used is since, which applies to the deprecated stability level.

Comparison with Weaver

In Weaver V2, deprecation is modeled differently: the stability group no longer includes a deprecated level. Instead, deprecation is represented in a separate group called deprecated, making the stability field structure simpler and flatter.

Proposal

Adopt this approach in metadata.yaml

Future Work

We should make similar updates to other parts of the metadata.yaml schema to align more closely with Weaver. Eventually, this will allow a full migration to the Weaver model.

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