HTTP migration, Env Var name

[MadVikingGod]

This change renames the environment variable used for compatibility mode to OTEL_SEMCONV_STABILITY_OPT_IN as per the recommendation in the spec.

Follow up from this comment.

[MrAlias]

cc @VinozzZ

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.5%. Comparing base (bb9ac54) to head (07ad52a).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #5997   +/-   ##
=====================================
  Coverage   65.5%   65.5%           
=====================================
  Files        203     203           
  Lines      12918   12918           
=====================================
  Hits        8466    8466           
  Misses      4198    4198           
  Partials     254     254           

Files	Coverage Δ
...entation/net/http/otelhttp/internal/semconv/env.go	88.6% <100.0%> (ø)