otelrestful: use semconv package for request attributes and span status

[dmathieu]

We missed this use of semconvutil which we need to remove to be able to finalize the semconv migration.

I chose not to add a changelog entry here because there is technically no visible change, since the default is still to use the old semantic conventions only.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.0%. Comparing base (216875f) to head (0486a84).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #6951   +/-   ##
=====================================
  Coverage   80.9%   81.0%           
=====================================
  Files        204     204           
  Lines      18065   18065           
=====================================
+ Hits       14627   14633    +6     
+ Misses      3011    3005    -6     
  Partials     427     427           

Files with missing lines	Coverage Δ
...hub.com/emicklei/go-restful/otelrestful/restful.go	100.0% <100.0%> (ø)

... and 3 files with indirect coverage changes

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[dmathieu]

cc @XSAM for review.