Switch the semconv migration default to new

[dmathieu]

This switches the semantic conventions migration default from the old version to be the new one, and keeps the possibility to use http/dup to get both.

Note: we previously tested and documented an old config.
That worked because it used the default. However, that behavior is not documented in the semconv migration spec, and allowing to keep using the old values only when the next release will entirely drop their support looks like a bad idea.

This also removes the v1.20.0_test.go files. Those tests rely on the global server/client, but only tests the old behavior.
We can't run the old behavior only anymore, so running those would require a non-trivial refactoring that seems unneeded since this entire code will be removed in the next release.

Closes #6897

[codecov]

Codecov Report

Attention: Patch coverage is 95.75758% with 7 lines in your changes missing coverage. Please review.

Project coverage is 80.7%. Comparing base (56eeab7) to head (c1b8859).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...tp/httptrace/otelhttptrace/internal/semconv/env.go	93.9%	2 Missing ⚠️
...entation/net/http/otelhttp/internal/semconv/env.go	93.9%	2 Missing ⚠️
...lei/go-restful/otelrestful/internal/semconv/env.go	96.9%	1 Missing ⚠️
....com/gin-gonic/gin/otelgin/internal/semconv/env.go	96.9%	1 Missing ⚠️
...ub.com/gorilla/mux/otelmux/internal/semconv/env.go	96.9%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #6899     +/-   ##
=======================================
- Coverage   81.0%   80.7%   -0.3%     
=======================================
  Files        205     205             
  Lines      18098   18083     -15     
=======================================
- Hits       14667   14604     -63     
- Misses      3000    3060     +60     
+ Partials     431     419     -12     

Files with missing lines	Coverage Δ
...lei/go-restful/otelrestful/internal/semconv/env.go	90.9% <96.9%> (+1.8%)	⬆️
....com/gin-gonic/gin/otelgin/internal/semconv/env.go	94.4% <96.9%> (+1.9%)	⬆️
...ub.com/gorilla/mux/otelmux/internal/semconv/env.go	92.3% <96.9%> (+1.8%)	⬆️
...tp/httptrace/otelhttptrace/internal/semconv/env.go	89.5% <93.9%> (+1.8%)	⬆️
...entation/net/http/otelhttp/internal/semconv/env.go	89.5% <93.9%> (+1.8%)	⬆️

... and 12 files with indirect coverage changes

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[AyseGokmen]

The new convention http.client.request.duration.seconds might still be recording the metrics in milliseconds and with the histogram buckets that would fit the milliseconds use case. Can we please update them as well when switching the default to the new convention?

[dmathieu]

Thank you. I have extracted your report into #6941

[MrAlias]

@XSAM PTAL