mdatagen: fix deprecated metrics validation and align metadata schema

- Fix deprecated metrics validation logic
- Enforce consistency between stability and deprecated fields
- Align generated output with updated metadata schema
- Improve test coverage for Stability.Unmarshal

Author: Syedowais312

.chloggen/fix_mdatagen-deprecated-metrics.yaml

@@ -4,7 +4,7 @@
 change_type: enhancement
 
 # The name of the component, or a single word describing the area of concern, (e.g. receiver/otlp)
-component: cmd/metadata
+component: cmd/mdatagen
 
 # A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
 note: "Introduce additional metadata (the version since the deprecation started, and the deprecation reason) for deprecated metrics."