otelconf: support OTEL_CONFIG_FILE as it is no longer experimental

[codeboten]

This follows open-telemetry/opentelemetry-specification@2709433

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.1%. Comparing base (a64a103) to head (fcf8721).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #8639   +/-   ##
=====================================
  Coverage   82.1%   82.1%           
=====================================
  Files        182     182           
  Lines      13777   13780    +3     
=====================================
+ Hits       11319   11322    +3     
  Misses      2054    2054           
  Partials     404     404           

Files with missing lines	Coverage Δ
otelconf/config.go	89.3% <100.0%> (+0.3%)	⬆️

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