otelmongo: default to stable semconv

[dmathieu]

otelmongo has had support for both 1.21.0 and the stabilized versions for quite a while now (4 releases, over a year: 04b152d).

So this switches the default behavior of the module to use the stable semconv.
Older semconvs can still be used with the OTEL_SEMCONV_STABILITY_OPT_IN=database/dup environment variable.

This matches what we did for the HTTP libraries.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.1%. Comparing base (6544466) to head (afdf7a5).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #8230   +/-   ##
=====================================
  Coverage   82.1%   82.1%           
=====================================
  Files        195     195           
  Lines      13680   13678    -2     
=====================================
+ Hits       11237   11241    +4     
+ Misses      2037    2031    -6     
  Partials     406     406           

Files with missing lines	Coverage Δ
.../mongo/otelmongo/internal/semconv/event_monitor.go	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

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