Switch to stable code semconv attributes for 3.0

[SylvainJuge]

#13860 adds the ability to produce both stable and unstable semantic conventions for code.* semconv attributes by using the otel.semconv-stability.opt-in configuration option with either code or code/dup values. By default the unstable semconv attributes are used.

Switching to stable semantic conventions by default and removing support of unstable attributes is a breaking change that we plan to do in 3.0.

The implementation of this requires to partially revert/inline the changes made in #13860, here is a suggested list of steps for an "easy" refactor:

• remove usages of SemconvStability for code attributes so only stable attributes are produced
• remove unused code in SemconvStability and/or delete the class if no longer used for stable db semconv
• inline test assertions of SemconvCodeStabilityUtil in test code and remove the class.
• remove usages of otel.semconv-stability.opt-in with code or code/dup in project build files to remove testing for those