Don't allow signal definition to reference attributes with lower stability than the signal itself

[lmolkova]

(unless attribute is opt-in)

Currently we only check stable groups, but in #3693 we marked a metric as stable leaving some of its attributes as development - #3739

Let's extend policy to cover all stability levels.

[Copilot]

Pull request overview

This PR tightens the repository’s policy enforcement so that semantic convention signal groups (spans/metrics/events/entities, etc.) cannot reference attributes that have a lower stability than the group itself—unless those attributes are opt_in. This addresses cases where a signal was promoted but some referenced attributes remained less stable.

Changes:

• Extend the Rego policy from “stable-only” checks to a general stability ordering check using a stability ranking map.
• Update and expand policy tests to cover the generalized stability comparison behavior.
• Update documentation to describe the rule at the group level, and add a changelog entry for the change.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
policies/group_stability.rego	Implements stability-rank comparison and adds a new exception.
policies_test/group_stability_test.rego	Refactors/extends tests for the new policy behavior.
docs/general/semantic-convention-groups.md	Documents the new “no lower-stability attributes unless opt_in” guidance.
.chloggen/group-stability-check.yaml	Adds a changelog entry describing the enhancement.