[SDK] ResourceBuilder AddDetector public API tweak

[CodeBlanch]

Relates to #3782

Changes

• During API review for 1.4 release we decided to make this API internal.

[codecov]

Codecov Report

Merging #3997 (20b1723) into main (723ad0b) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3997      +/-   ##
==========================================
- Coverage   85.40%   85.39%   -0.01%     
==========================================
  Files         289      289              
  Lines       11228    11236       +8     
==========================================
+ Hits         9589     9595       +6     
- Misses       1639     1641       +2     

Impacted Files	Coverage Δ
src/OpenTelemetry/Resources/ResourceBuilder.cs	89.79% <ø> (ø)
...ation.SqlClient/TracerProviderBuilderExtensions.cs	100.00% <100.00%> (ø)
...ZPages/Implementation/ZPagesExporterEventSource.cs	56.25% <0.00%> (-6.25%)	⬇️
...p/Implementation/HttpInstrumentationEventSource.cs	72.00% <0.00%> (-4.00%)	⬇️