fix(browser-detector): set user_agent.original resource attribute always

[david-luna]

Which problem is this PR solving?

As a result of the discussion in the Browser SIG it is decided that the user agent should be set in the resource. This PR
aligns with the changes proposed in semantic conventions in open-telemetry/semantic-conventions#3738

Short description of the changes

• set the user_agent.original resource attribute always

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Unit tests

Checklist:

• Followed the style guidelines of this project
• Unit tests have been updated

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.95%. Comparing base (6496dbb) to head (c448639).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6754      +/-   ##
==========================================
- Coverage   94.95%   94.95%   -0.01%     
==========================================
  Files         377      377              
  Lines       12744    12743       -1     
  Branches     2897     2896       -1     
==========================================
- Hits        12101    12100       -1     
  Misses        643      643              

Files with missing lines	Coverage Δ
...ntelemetry-browser-detector/src/BrowserDetector.ts	100.00% <100.00%> (ø)

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