docs(opentelemetry-browser-detector): Use tree-shakeable string constants for semconv in Readme

[JohannesHuster]

Which problem is this PR solving?

Updates #4567

Short description of the changes

Replace deprecated imports from @opentelemetry/semantic-conventions with new (tree-shakeable) string constants for package @opentelemetry/opentelemetry-browser-detector. What I did:

• Search for “@opentelemetry/semantic-conventions” in package path.
  • Add new imports and check that new and old strings match exactly.
  • Use new imports and remove old ones.

Type of change

• [x] Bug fix (non-breaking change which fixes an issue) (refactor)

How Has This Been Tested?

• [x] npm test

Checklist:

• [x] Followed the style guidelines of this project
• [ ] Unit tests have been added
  • No new functionality
• [ ] Documentation has been updated