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 75.2%. Comparing base (9fae37d) to head (ec5b228).

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   75.5%   75.2%   -0.3%     
=======================================
  Files        207     207             
  Lines      19319   19304     -15     
=======================================
- Hits       14601   14535     -66     
- Misses      4283    4343     +60     
+ Partials     435     426      -9     

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 13 files with indirect coverage changes

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