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 80.7%. Comparing base (a50074d) to head (b15c691).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #6951   +/-   ##
=====================================
  Coverage   80.7%   80.7%           
=====================================
  Files        205     205           
  Lines      18083   18083           
=====================================
+ Hits       14601   14604    +3     
+ Misses      3062    3058    -4     
- Partials     420     421    +1     

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

... and 2 files with indirect coverage changes

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[dmathieu]

cc @XSAM for review.